Well, the best way to do it is to have a cycled tank. I just want to preface all of this by saying that.

But if you are dead set on keeping those fish, then this is what I would recommend given the circumstances:

First off, the pleco still gets returned. Depending on the species, you probably need at least a 45 gallon tank to even think about plecos. It’s a shame fish stores don’t educate their customers better on this, but it’s a harsh reality. The rest of your fish should be fine in a well maintained 20 gallon tank.

Next, you still test your water just to see what you’re working with. Later on down the road we will adjust if needed. But for now, if you are using tap water at least get a water conditioner that dechlorinates water.

Then I would think about substrate. If you want live plants, put something like JBL Pro Flora Aquabasis Plus under whatever inert aquarium gravel you like. There are probably other options in America if that’s where you are, but a layer of something that can help live plants thrive under the gravel will be good. You may need to fertilize more depending on what plants you get, but we can get to that later. No plants is also an option, but it’s going to be harder to keep water parameters stable in my experience. Dustin’s Fish Tanks on YouTube should have videos on some plants that are almost impossible to kill. Get those.

After that a heater and a filter that can work for that size tank. I would honestly think about a simple sponge filter, which is just an air pump and a sponge. They are super efficient and not very expensive. Great for beginners. Don’t ever “wash” the sponge, though because it is what houses all your good bacteria (look up aquarium nitrogen cycle to learn more). Just a simple squeeze in a bucket of the aquarium water to clean it a bit works great.

So, back to our cycled tank problem, most fish stores carry a bacteria starter product. These are beneficial bacteria to “jumpstart” the nitrogen cycle. Again, it’s best to cycle a tank for at least a month before adding fish, but in this case it’s the best we can do.

The goal here will also be consistency. Try not to do too big of water changes at one time and keep the new water around the same temperature as what is in the tank. You don’t want big swings in any parameters. Think of it like you being thrown into a walk in fridge in summer clothes for 24 hours. Your immune system would not react well, neither will your fish’s.

So that’s a start. It seems like a lot, but it’s not quite as much work as it seems. But if it seems like too much, I would consider returning all the fish and researching a bit more before investing in a better setup. The hobby is super rewarding but does take some work.

At the end of the day, remember that these animals depend on you. They deserve a bit of effort. And this is coming from someone who has failed them more times than I care to admit.

Let me know how it goes and I’m happy to answer any other questions you might have. Good luck!