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[mpgt]

Tracy, (sorry for the long post)

You should do an Internet search for “new tank syndrome” or “tank cycling”. I think your cloudy water is from ammonia, which is pretty toxic to fish. The ammonia comes from a bloom of anaerobic bacteria, which is a predictable, normal step in a new tank’s process of getting established. (This is why you never “clean” an established tank or filter too thoroughly.) Once the beneficial aerobic bacteria take over in sufficient numbers the anaerobic bacteria and the ammonia will disappear.

You should remove the fish if you possibly can. Catfish and algae eaters (if he’s a pleco) are hardy, but they have their limits too. The tank will cycle on its own if you get them out and leave that dirty water in there. I would remove the carbon until the tank is cycled. There’s good reason for that. You could speed up the process a lot by “seeding” the gravel bed with bacteria laden gravel from an established tank, or even putting an air driven corner filter full of it in the tank.

If you must keep the fish in there (not the best option):
You should do some water changes every couple of days (maybe 25% at a time). Bottled, deionized water might be best right now. I would even use tap water as long as it’s the right temp. Chlorine is not a terrible thing but you need to find out if your water company adds chloramines. If so, that’s another problem. If you can find some zeolite (some pet stores sell it for cat boxes. A few carry it for fish, but it's the same thing), add it to the filter or even put it in a mesh bag (well rinsed until the water runs clear) on the bottom of the tank. It absorbs ammonia.

The ammonia makes it hard for the fish to breathe, and they might eventually die from asphyxiation. You can help by adding an air stone or 2.

A 20-gallon tall will not be an easy tank to maintain. Any “tall” tank will have less surface area proportionately, compared to another tank of the same capacity. One of your goals is to get oxygen into the water and remove carbon dioxide (CO2). A filter, the bio wheel, an air stone in the water will all agitate the water surface. The water surface is where over 95% of that O2/CO2 transfer happens. If you add an air stone or bubble wand to the tank with an air pump, the bubbles rising to the surface “move” the water for you and agitate the water surface for you, thereby enabling the O2 transfer.

Also, the 20-gallon is small. The smaller the tank is, the less stable it will be, and the harder it is to maintain. If a fish dies and isn’t removed right away, or a little uneaten food decays, it can quickly upset the balance and put you back into the position you’re in today. It can work of course, but I think you should be on the lookout for a bigger tank.

You have a great filter, but you really could use a larger version.

How deep is your gravel bed? I’d keep it under ½” so it doesn’t accumulate too much anaerobic bacteria or dirt, and produce lots of nitrates. You could use rocks or little walls to create a few deep spots if necessary for plants.

Again, sorry for the long post. There's much more to say but I'll wait to see if you have any questions.

P.S. Once you get things right, you should change about 10% of the water (siphoning the gravel with a python) once a week.