I have a 6 gallon tank. 2 months ago I had 4 fish: a cory catfish, a female betta, and 2 otos. They had all been in the tank for a year and were doing fine.
Three weeks ago, I found the Cory dead in the bottom. He had been acting lethargic. Now my betta just sits on the bottom and seems unhappy. Otos seem fine.
I always do a weekly 50% water change and treat the new water with a dechlorinator before pouring it in. Tank is kept between 79-81 degrees. PH is high, but that never bothered the fish before and people have told me to leave it alone. I feed them every other day with either blood worms, brine shrimp or flake food. Pet sitter fed them just flake food.
One thing I noticed when I tested the water yesterday is that the ammonia was kind of high, especially for an established tank. Nitrate and nitrate are zero. Last test showed zero ammonia and nitrite, very low nitrate. Not sure what happened. Did a 50% water change. Didn’t seem to help. Don’t see any signs of ich or parasites.
Also, I went on vacation for a week a couple months ago and the pet sitter overfed them. Came back and the tank was filthy. I did a 65% water change and siphoned as much food out as possible.