Clean fresh water is what is in order, and treated water with a quality product like Prime, which is a little more expensive, but highly concentrated.
Usually a chemical wouldn't be necessary if you can do lots of partial water changes(pwc).
Fresh clean water will not need the chemical. Check your tapwater for ammonia. Occasionally it comes from the tap that way, I had that experience before.
I would do 50-75% pwc, then that evening or the next morning another 50%, check the ammonia, nitrite and nitrate levels too
Its like swimming in pee water which keeps getting more toxic, and just because you put a chemical to lessen the toxic effects doesn't mean you still aren't swimming in pee water. (not an exact scientific explanation, but I hope you get the point) So fresh water is much better.
Besides what are the Nitrite levels? Very toxic to fish as well.
Redtail sharks get large and although they are really cool looking and are sold to everyone with smaller tanks, they shouldn't be in one for very long. So many of the commonly sold fish, don't belong in smaller tanks (like 25G), like silver dollars, and goldfish and Oscars and bichirs and many kinds of, including the common Plecostomous. You can't rely on the fish store people to steer you in the right direction, do lots of research.
Have you read about fish in cycling. Did you save your filter pad from your broken filter or start with a new one? You can rinse off the filter pads with treated or old tank water, but using tap water which has chemicals to kill bacteria, does just that kills bacteria in your filter pad, the beneficial bacteria (BB) which help convert ammonia and nitrites to nitrates. Go to the "getting started" section and there are some resources to check up on. You can do a search on here, in "getting started"
Work towards getting the water quality in check, try to get Prime it helps with ammonia also, chances are your lfs has it, I know Petco does. You can deal with the stocking issues after or while you get the water better.
Also chances are you are over feeding and you should feed a high quality food to help reduce fillers which don't do anything for the fish or the water quality. Try feeding half of what you normally would too. It is very easy to over feed. I don't know positively that you are doing it, but it happens most of the time.
Keep us posted.
