This is actually really sad because we have alot of replies but no one here is really stressing the severity of the situation nor is the op getting how to fix the situation as too many suggestions when there is only one thing to worry about right now.
YOUR FISH ARE CURRENTLY DYING THROUGH UNFIT LIVING CONDITION BY THE MEANS OF TOXIC WATER.
You need to act immediately!!
Step 1. Change as much water as you can. This mean very large water changes daily. But it's super important that you maintain stable temperature during this process. You need to change alot of water regularly to remove ammonia and or nitrites to close to 0 as possible. Yours aquarium is not cycled, this means you have no beneficial bacteria which is what converts ammonia to nitrites to nitrates.
Nitrates are safest form although still toxic at higher levels and each fish has a different degree of sensitivity to nitrates. This is why we change water weekly or more in our aquarium.
Don't forget to use a quality water dechlorination product like seachem prime. You must add this to the tank to ensure your removing chlorine and heavy metals.
Step 2. Now that the water is suitable for life you need to quickly cycle the aquarium to ensure ammonia and nitrite are being converted properly and not have to rely on removing them through water changes each day.
The fastest way to do this is to ask a store or fellow hobbiest to provided with seeded media from an established aquarium. Add this to your filter and you can essentially instantly cycle the aquarium.
I would also highly suggest that you stop feeding for a few days or more untill things get sorted out to avoid adding additional waste which will only cause spike of ammonia in the aquarium risking your fish.
Also a good idea to add a bottled product that will provide additional bacteria. ATM colony is one of these that works. Dr Tim's is another.
https://www.amazon.ca/DrTims-Aquatics-Nitrifying-Bacteria-Cycling/dp/B002DVTFGE
Adding this will definitely help to establish a proper bacteria colony to deal with the bioload of the aquarium.
If you can quickly get on this you should start to see improvement in your fish pretty fast. Like almost immediately. The key is stop feeding and get the water free from ammonia and nitrite a which are deadly to fish.
Start changing large volumes of water each day 8f not twice a day till you see you fish improving. Get some seeded media combined with a nitrifying bacteria in a bottle product right away and add this to the tank and follow directions carefully.
I would keep lights off during this time to reduce stress.