Oh boy. From the sounds of it you're treating your discus like regular fish. You can't do that. Especially when they are as small as yours. You'll have to radically change things up or they will continue to get worse and die.
I'll tell you what you need to do, but you won't like it. I'll give it to your straight without sugar coating. But be warned, it is a lot of work.
First off, you can't keep discus if you have less than 6. They are schooling type fish, and need at least 6 or they don't feel safe, and become stressed. They will also pick on the runt.
You need at least 10 gallons/40 liters per fish. So if you have less than a 60 gallon tank, don't bother with discus. That's only for the discus too. No other fish in the tank.
Keep the temp at least 82f/28c if your temp is cooler, their immune system is not up to par, and they get sick.
You need to feed at least 6 times per day. They are fish in warm water, their metabolism is revved up. They are also fish with mostly body, so they need food. Especially when they are juveniles like yours.
With all that feeding, you have a lot of food waste, and also fish waste. Discus again are not like regular fish. They need pristine water and tank conditions. They will nit tolerate nitrates. Never mind nitrite or ammonia. They need zero ammonia, zero nitrite and nitrate less than 5 ppm.
You can't leave your filter uncleaned that long. It'll become a nitrate factory. You need to keep it clean. Put some sort of pre filter on the intake tube in your tank. It'll keep some of the junk out of the filter. But you'll need to clean that prefilter daily. Clean out your filter. But only use tank water when you do.
Now to the work part. You will have to remove the substrate. All of it. The tank needs to be bare bottom glass. Until they are adults. Meaning until they are over 6 inches. You'll never be able to keep the tank clean enough otherwise.
You need to do huge water changes daily. Yes I mean daily! And by huge I mean huge. Like 90% of the water volume. Maybe even twice per day. Go to youtube and watch the videos of how the breeders do it. They drain all the water. Until the fish flops on the bottom of the tank sideways. Then they refill. They do that twice per day. You can probably get away with 90% though, so the fish don't have to flop. Check your chemistry. Again you need less than 5ppm nitrate. Clean all sides of the tank and bottom daily with paper towels to remove all junk.
Get quality food. They need high protein food. You can't use regular fish food from walmart, and not just one type, they need variety as well.
The key is the water quality. You need to change the water daily. That is 90% of the equation with discus. Do that alone and you'll probably see them improve.
Good luck. Continue to report back for help.
I'll tell you what you need to do, but you won't like it. I'll give it to your straight without sugar coating. But be warned, it is a lot of work.
First off, you can't keep discus if you have less than 6. They are schooling type fish, and need at least 6 or they don't feel safe, and become stressed. They will also pick on the runt.
You need at least 10 gallons/40 liters per fish. So if you have less than a 60 gallon tank, don't bother with discus. That's only for the discus too. No other fish in the tank.
Keep the temp at least 82f/28c if your temp is cooler, their immune system is not up to par, and they get sick.
You need to feed at least 6 times per day. They are fish in warm water, their metabolism is revved up. They are also fish with mostly body, so they need food. Especially when they are juveniles like yours.
With all that feeding, you have a lot of food waste, and also fish waste. Discus again are not like regular fish. They need pristine water and tank conditions. They will nit tolerate nitrates. Never mind nitrite or ammonia. They need zero ammonia, zero nitrite and nitrate less than 5 ppm.
You can't leave your filter uncleaned that long. It'll become a nitrate factory. You need to keep it clean. Put some sort of pre filter on the intake tube in your tank. It'll keep some of the junk out of the filter. But you'll need to clean that prefilter daily. Clean out your filter. But only use tank water when you do.
Now to the work part. You will have to remove the substrate. All of it. The tank needs to be bare bottom glass. Until they are adults. Meaning until they are over 6 inches. You'll never be able to keep the tank clean enough otherwise.
You need to do huge water changes daily. Yes I mean daily! And by huge I mean huge. Like 90% of the water volume. Maybe even twice per day. Go to youtube and watch the videos of how the breeders do it. They drain all the water. Until the fish flops on the bottom of the tank sideways. Then they refill. They do that twice per day. You can probably get away with 90% though, so the fish don't have to flop. Check your chemistry. Again you need less than 5ppm nitrate. Clean all sides of the tank and bottom daily with paper towels to remove all junk.
Get quality food. They need high protein food. You can't use regular fish food from walmart, and not just one type, they need variety as well.
The key is the water quality. You need to change the water daily. That is 90% of the equation with discus. Do that alone and you'll probably see them improve.
Good luck. Continue to report back for help.