Without being able to
test the water for
ammonia and
nitrites, it’s impossible to say if your tank is still properly cycled for the amount of fish you have.
I recommend you immediately go to a pet store/fish store and get
Seachem prime. Dose it every 24 hours (the Seachem forums keep saying it works 24-48 hours, so I would dose every 24 to be safe. It all eventually leaves the tank, so you don’t have to worry about buildup).
Also add Seachem prime every time you
change water (if you are on municipal water, your water has
chlorine or Chloramine in it, which is bad for your aquarium. Seachem prime will get rid of them). (Always add per instructions - so every 24 hours add however much is supposed to go in 20 gallons, and if you do a 50% water change, add however much is supposed to be added for 10 gallons. Don’t worry about overdosing, you can easily add at least 5x a normal dose, as per manufacturer instructions)
Also, I recommend you get the API master test kit (you want to be able to test for
pH, ammonia, nitrite, and
nitrate. Also - it’s good to be able to test for
KH and GH, but you’ll have to buy those separate).
If your fish are dying, and you don’t know why, a good place to start is to keep the following parameters:
Temp - between 70-80F (and make sure when you do water changes you use a
thermometer to match the water temp out of the tap with the temp in the tank, give or take a couple of degrees)
pH - between 7.0 and 8.0 (you can buy acid
buffer and
alkaline buffer to manage KH and pH. OR - if your pH is below 7, most likely changing enough of the tank water with
tap water will bring the pH above 7. Your nitrifying bacteria don’t like acidic water. It’s important to try and keep the pH stable, so if you notice the pH changing a lot, you either need to do frequent water changes or increase the KH of your water and monitor the KH + pH)
Nitrates - under 20 (change the water at least 25% if it get higher)
Ammonia and Nitrite - change the water 25-50% if either gets over 5ppm.
Ammonia and Nitrite - if you detect nitrite, then your nitrifying bacteria haven’t caught up to the bioload of all the fish. Administer Seachem prime every 24 hours until there is no detectable nitrite. You can change the water when you get to 5ppm nitrite, but you need to leave SOME nitrite in the water, otherwise your nitrite eating bacteria won’t have food to eat, and won’t be able to establish
Ammonia is a little tricky - when Seachem gets rid of Chloramine, it creates ammonia (if your tap water has 2ppm Chloramine, you’ll immediately get 2ppm ammonia when you dose Seachem prime). So, just because you detect ammonia right after you do a water change and dose with prime, that doesn’t mean you have a problem with your ammonia eating bacteria. However, in your case right now, I would do the same with ammonia as with the nitrite - don’t change the water unless it gets over 5ppm ammonia (like with nitrite, you don’t want to change the water so much that you constantly bring it down to 0 - you want to leave food for the bacteria to eat. They will bring it down to 0 on their own once they multiply enough). Dose with prime every 24 hours.
Like the others said - be careful with overfeeding. Try giving them only what they can eat in a couple of minutes, once a day.
If their stomachs start to look sunken in after a week, feed a bit more.
Oh - also.
Siphon out some tank water into a bucket, and swish around the sponge inside your filter to clean it off a bit (not too much - you are just trying to knock off particles that are caught in the filter, but you want to keep all the nice slimy bacteria).
