I use Kordon Amquel Plus in this tank every day. I use 3-4 drops which is about the ratio of 1 tsp per 10 gals. I did the drop count for 1 tsp and it was about 35 drops. Divided it by 10 to get the dose for 1 gal. Then I was changing the water once every 5 days. His bubbler is under gravel and creates an under gravel type filter. When I change his water, I use the gravel vacuum and clean the top waste. I've used a little of the bio sludge from my 20 gallon tank in his tank to help age the water. I don't change all the water,just about 3/4th of it. I feed him one pellet at a time and give him a few minutes between pellets. When he starts to play with his food is when I stop. Usually about 3 to 5 pellets. He is very skinny and I feel like I am not feeding him enough but the pet store people told me not to feed him too much. Am I doing something really bad?
By the way, yes I changed his water just before seeking help. While he was waiting I had him in the aquarium. same ph as his tank. More filtration systems and aged water.
Hope the fish recovers. But just to make sure, changing 3/4th of the water in 5 days is just asking for an inevitable disease or death if there's no filter. You definately need to do that every day for a 1G tank. And if you have been doing the same maintenance on the rest of the bettas, they'll start dying one by one early or later, depending on how strong their immune system is, but they'll give in eventually. This one is just the weakest of them so he's gone downhill first. The fish produces ammonia not through poop only, but through the gills/skin and there's no ammount of vaccumming that will stop the water from becoming toxic with ammonia, unless you install a filter and cycle the tank. If you have so many fish, invest in a liquid test kit like API brand for example. Test for ammonia and nitrItes in all tanks, if any of them are not 0, then fish are being slowly poisoned. For fish living in these conditions, it's like breathing their own pea, with that difference that the pea is burning them outside in and will eventually cause irreversible damage, disease and slow and painful premature death.