I have 2 29 gallons, one has 3 tiger oscars, 2 kenyi cichlids, 2 pictus catfish, a brown bullhead, and 1 pleco. It has been running for almost 7 mos now, the last 2 times I checked the ammonia it was a pale green without the water change (surprising me alot, cause it's usually dark green). I did a small water change before we went away this weekend and I checked it last night and it was really dark so I did atleast a 50% change. (I know I will need a bigger tank, but I mentioned that above already). I've had the same fish together for at least 5mos, and if I try to add a fish(evan a big one) they try to kill it.
My other has an albino channel cat, 1 rainbow shark, 1 bala shark, 1 columbian catfish, 1 baby bullhead catfish, 1 banjo catfish, and a pleco. I do wish I had researched these fish before I bought them, a couple of them get pretty large I guess *thumbsdow . That tank has been up for 2 months now and has gone through a couple of bacteria blooms I guess they call it, just like the first tank did, but the ammonia and nitrite are usually around 0, and I do a small water change once a week at least. *BOUNCINGS