How to Setup a 20 Gallon Fish Tank

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If you’re looking to get into the fish keeping hobby a 20 gallon fish tank is the perfect place to start. Setting up a little 20 gallon tank takes a little bit of time and effort but it’s not as complicated as it can seem. I upgraded my very first 10 gallon tank to a little 20 gallon tank a few years ago so I thought I would show you all the pieces and parts that make this little guy work.

If you’re researching how to set up a 20 gallon aquarium chances are you are setting up a brand new fish tank. While I do have two larger 55 gallon fish tanks when I went to set up this 20 gallon fish tank I needed all new supplies (since the 10 gallon supplies were not strong enough for my new 20 gallon… also how many times in a paragraph can you say “20 gallon fish tank” before it’s completely over-rated?)… so I was thrilled when my brother surprised me with this little 20 gallon hexagon tank for my birthday!

How much does a full 20 gallon long aquarium weigh?

So let’s get right into it. If you’re bringing home a fish tank of any size you need to be aware of how much the thing is going to weigh. Let’s face it… fish tanks are heavy but water is heavier. Luckily this is basic math… literally.

First things first: find the dimensions of your new fish tank. Then it’s simply L x W x H. A standard 20 gallon long aquarium is 30″ x 12″ x 12″ so your total weight is 225 lbs. That’s just a couple pounds y’all!

You definitely don’t want to place your brand new fish tank on something that can’t hold it’s weight… the whole thing is likely to come falling down! My 20 gallon fish tank is currently resting on a sturdy solid wood dresser in my bedroom… the new prefab dresser across the room?

Let’s just say I would not trust the prefab dresser with the weight of any fish tank… but especially not one that weighs 225lbs. If you don’t have a piece of furniture sturdy enough to support your new fish tank an aquarium stand is the perfect solution (both of my 55 gallons are on stands).

Either way let’s get started with all the things you’ll need!

What Fish Tank Should I buy?

First thins first… let’s pick a fish tank. I absolutely love aquariums that are slightly unusual sizes but they can be a little hard to find… the hexagon kit my bro originally snagged for me is no longer available. Keep an eye out though since it comes back in stock every now and then… in the meantime this 20 gallon aquarium starter kit is a great price and comes with everything you need to get started.

The best part about a fish tank starter kit like my brother bought me is that everything you need is already in the box… this little tank even came with a fish net and thermometer! Of course, many of us in the fish hobby already have bits and bobs from other tanks… so here’s my list of supplies when you’re putting together a brand new fish tank.

Fish Tank Supplies:

  • 20 Gallon Fish Tank (Starter Kit or Empty Tank)
  • 20 Gallon Stand
  • Fish Tank Lid (in Starter Kit)
  • Day/Night Tank Light (this is the one I have… it has a remote for night time!) (In Starter Kit but without the remote)
  • Water Filter (in Starter Kit)
  • Water Heater (in Starter Kit)
  • Substrate (Sand, Gravel or None)
  • Plants and Decorations (Optional depending on the fish)
  • Air Pump (Optional)
  • Bubbler Ornament (Optional)
  • Net Tool (in Starter Kit)
  • Siphon
  • Water Conditioner
  • Water Master Test Kit

BONUS TIP: When picking a tank keep in mind the size of the fish you want to put in it… you can’t put a 6 inch fish in a 5 gallon tank. Of course, there are lots of factors that go into which fish can live in what size tank (we’ll get into that in a moment) but for now let’s just assume that you’re setting up a basic 20 gallon fish tank.

How Many Fish can fit in a 20 Gallon Fish Tank?

Alright y’all let’s get down to business. The basic answer is that it depends! Your water quality, tank makes, filtration, oxygen… all of these things play a big part in how many fish you can put in your new fish tank. BUT a general rule of thumb for newbies is 1 inch of fish for 1 gallon of water.

Which means the most you could have in a 20 gallon tank is 20 one inch fish.

Don’t forget to take into account the full grown size of your fish

That cute little 1 inch fish at the pet store may grow to be a foot and a half. Been there, done that, got the t-shirt (and a bigger tank for that monster to grow up in).

Most fish stores have adult sizes on their tanks but when in doubt ask… or even better look it up! Not all fish store employees are created equal (I bought 3 fish that were full sized instead of dwarf sized when I was starting out at the recommendation of an employee… these fish would grow to be 6 inches each instead of 2… yikes!). My brother (the fish dude) recommends checking for info… so that’s what I do now.

So for example say you want to stock your fish tank with tetras. A nice schooling fish that is easy to keep alive. A black skirted tetra typically grows to about 2 inches in size so in a 20 gallon tank you could have 10.


It’s a guideline, not a rule: 1 inch of fish for every gallon of water

Just remember to do your research on the specific type of fish you are getting and their tank requirements. Experienced fish peeps will tell you the 1 inch per gallon rule is crap as alllllll of the other factors play a big part but it’s a good starting point.

Basically the more you know the more you can do… experienced fish people know what they’re doing and can handle the bio-load of an over-stocked tank! I can’t. I stick to the 1 inch per gallon rule. I know how to over-stock a tank but I’m busy y’all… I can’t keep up with the cleaning of an over-stocked tank!

What Kind of Fish can you Put in a 20 Gallon Aquarium?

Now we’re getting to the good stuff! What fish go good together in a 20 gallon fish tank? Of course, there are a million combinations… and a million different types of fish!

While there are literally a million trillion bazillion fish in the universe I’m only going to list my favorite beginner fish that you can mix and match to stock your fish tank. Search for them on so that you can see what they look like, what their adult size is and what their best tank mates are.

The possibilities are endless!

Fish For a 20 Gallon Aquarium:

  • Guppies
  • Platies
  • Dwarf Gourami (in the photo above!)
  • Neon Tetras
  • Black Skirted Tetras
  • Blushing Tetras
  • Diamond Tetras
  • Dwarf Rasboras
  • Tiger Barbs
  • Zebra Danios
  • Mollies
  • Swordtails
  • Bristlenose Catfish
  • Cory Catfish

Led Fish Tank Lights

Let’s start getting our tank ready for fish! Starting with the most important component: light!

Okay there may be other important components for the fish but lighting is pretty important for the fish enthusiast… after all what’s the point of an aquarium if you can’t see the fish! Although there are plenty of lights available I love this Led light that provides a white light for day and a blue light for night.

They are bright and clear during the day and provide a nice blue light at night (below) so that you can still see the fishy’s but it’s less stressful for the fish. I love this specific light because it has a remote control… so I can turn it off from bed when I’m done reading for the night. It also has lots of other colors (like pink!) which I love although I’m not a fan of the strobe light setting. I just don’t under stand it… seems like that would give the fish little tiny fish seizures or something!

It’s best to turn your tanks off at night so the fish can have a bit of down time without a light on… it’s not great for the light to be on 24/7 even if they are in an office or living room and the light won’t effect you while you sleep.

Adding an Air Pump and Bubbler

While my little tank may not need an air pump as much as a larger tank (where the water is lacking oxygen) I do like to put one in each of my tanks. First of all they’re cool. Second of all they help add oxygen to the bottom of the tank when you have a tall tank like I do.

I simply picked up a small air pump and a bit of tubing to attach to my bubbler. There are plenty of bubblers to pick from (my brother has a cute little barrel bubbler in his 30 gallon) but I stuck with my pink and blue theme and picked out this cute little pink coral bubbler.

It’s hard to see in the photograph but the air travels from the little air pump in the picture up above through the tube and into the pink coral where it’s released in bubbles that travel up through the tank providing the water with oxygen. I did get a shot of this little guy in action in the video at the bottom of the post.

Fish Tank Water Filter

Alright here’s the important part… the filter. Your filter determines how clean your tank will be and therefore how healthy your fish will be. The fish actually like a certain amount of algae… that’s good bacteria and is healthy for the fish. Uneaten food, fish poop, dirty sand… all of that is not healthy and the filter helps to… filter it out of the water.

Basically the better your filter the healthier the fish and the less water changes you will have to do. Now that’s what I’m talking about.

If you grab the starter kit it will come with a hang-on-the-back filter like the one in the photo above. It works just fine and honestly I’ve never had a problem with the one that came with my starter kit. (Knock on wood)

Under Water Fish Tank Water Filter

But in my larger tanks I prefer these under the water filters… they are so much quieter! Plus if you purchase your filter seperately you can get a larger filter.

Basically you can never have too much filtration… so instead of a 20 gallon filter on a 20 gallon fish tank I’ll usually size up. So I’ll use a 30 gallon filter on a 20 gallon tank, a 20 gallon filter on a 10 gallon tank. You get the idea!

On my brother’s large 55 gallon he has two 75 gallon filters…. now that’s a lot of filtration! Keep in mind that different fish have different needs… he has fancy, cool fish in his 55 that need a lot of filtration.

My 55 is set up for angelfish which are not strong swimmers and can’t handle a normal filter. I have a 55 gallon sponge filter set up for my sweethearts and it works perfectly… I also have about 10 panda corydoras and a bristlenose catfish in my 55 which are bottom feeders and help keep the tank clean. While I have a small chinese butterfly loach in the 20 gallon to add to the water quality. Bottom feeders are definitely a great addition to any tank and help keep all the fish happy.

I’m fairly sure every single tank should have a bristlenose catfish to help keep it clean!

Perhaps the next time I do a big water change I will show y’all how the sponge filter works and why it’s AWESOME! I had a hard time finding info on setting one up when I was starting out but it works perfectly.

Adding a 20 Gallon Fish Tank Water Heater

Since I have tropical fish these babies like their water in between 76 and 80 degrees. Without a heater the tanks in our home stay about 70 degrees so a little water heater makes these guys very happy. It’s an easy install (seriously… just stick it to the wall with the provided suction cups and plug it in) and once you set the temperature doesn’t really require any maintenance.

Just make sure to turn it off during water changes since it really does need to be submerged when turned on and you’re good to go! If you’re looking to monitor it a little thermometer is the perfect solution and isn’t very invasive. Plus the fish seem to like looking at it… go figure.

Now you’re ready to add plants!

Plants, Decor and Sand to make your tank stand out

Most little fish like plants to swim around, caves to swim through, places to hide and typically a little decor to make their lives easier. Even schooling fish like tetras get along better when they have places to hide when need be.

Imagine if you were being chased around with no where to hide and no where to go. That would be horrible! Plus plants come in pink so……

This little 12 inch blue plant is one of my absolute favorites… the leaves are so pretty! You can see it peaking through the pink plant there in the back. I actually have roman columns in both of my tanks and love how the fish swim in and out of them… my alpha blue gourami has claimed one set as his territory in the 20 gallon. I caught a picture of him swimming around them this morning!

BONUS TIP: Live plants are amazing for your fish tank but they’re a whole new species to learn. I love using fake plants in my aquariums since they’re easy to maintain and come in fun colors. Here’s a list of my favorite 12 aquarium plants for beginners.

The last step is adding Water

Once your aquarium has sand, plants, a filter, a heater, a lid…. it’s time to fill it with water! Of course, it’s much easier to add the plants and decor once the tank is about half way full of water than when the fish tank is completely empty so don’t take those steps literally.

Although depending on your sand the water can get quite murky the first time you fill it and you may have to wait patiently for the water to settle.

I am not patient.

I hate cloudy fish tank water.

You’ll need to look into your specific fish and their water requirements but for my little guys tap water and a good water conditioner are all I need. I bought a bottle of water conditoner years ago and only recently reached the end… this stuff goes a long way!

When adding water to your aquarium pour the water onto a plate or flat surface instead of directly into the tank so that the water doesn’t disturb the plants and sand as much as possible. Of course, if you already have fish make sure they are safely out of the aquarium and in a bucket with water from the tank. Then once the water murkiness is gone you can add your fish!

BONUS TIP: If you’re adding brand new fish to a brand new aquarium you’ll need to let the tank cycle after you’ve added the water. This can take a week or two. Here’s a great article on aquarium cycling if you’ve never done it before. Or as I like to call it… the best way to NOT KILL YOUR FISH!

All that’s left now is to enjoy! Here’s a little clip of my fish as I was feeding them this morning. Good luck setting up your new tank… leave a comment below on what type of fish you are going to have! I always like to hear about other people’s pets!

20 Gallon Fish Tank Stocking and Setup

Psst… here’s a little sneak peek of my 55 gallon… I’ll definitely be sharing that with you soon!

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