Hi and welcome to the forum
First thing you do is put the filter back in the tank. Without a filter there will be an ammonia build up that will probably kill everything in the tank.
If you use medications, you need a filter in the tank to help break down the medication and provide oxygen. You should also only treat if you know what the problem is.
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BASIC FIRST AID FOR FISH
Test the water for ammonia, nitrite, nitrate and pH. If you don't have test kits, take a glass full of tank water to a pet shop and ask them to test it for you. Write the results down in numbers when they do the tests. If they say the water is fine, ask them for numbers.
Wipe the inside of the glass down with a clean fish sponge. This removes the biofilm on the glass and the biofilm will contain lots of harmful bacteria, fungus, protozoans and various other microscopic life forms.
Do a 75% water change and gravel clean the substrate every day for a week or until the problem is identified. The water changes and gravel cleaning will reduce the number of disease organisms in the water and provide a cleaner environment for the fish to recover in. It also removes a lot of the gunk and this means any medication can work on treating the fish instead of being wasted killing the pathogens in the gunk.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens so any medication (if needed) will work more effectively on the fish.
Increase surface turbulence/ aeration to maximise the dissolved oxygen in the water.
Post clear pictures and video of the fish so we can check them for diseases.
You can upload videos to YouTube, then copy & paste the link here.
