Gippicept plecos can attain lengths of 24" and should be kept in MONSTER tanks. They are even higher waste producers than the goldfish. I would give the pleco away as soon as possible. They are not necessary to keep algae away in tanks, and neither are snails. While they can help with algae, both need supplimented foods and both are high waste producers. Algae buildup can be due to too many dissolved solids in the water, often associated with higher nitrAtes and too much feeding....and it's also associated with light. While there are many other components of algae....your pleco isn't necessary and is adding to the problem.

Plecos are also very bad tankmates for goldfish because they are known to suck the slimecoat off the goldfish when the goldfish is resting or moving slowly. Fancy goldfish are especially susceptible to this because they move slower than the commons/comets.

It's important that you get a water test kit that can test for Ammonia, NitrAtes, Nitrites and PH. The strip test kits can be VERY unreliable so many of us use the API master test tube kit. It's more expensive at first, but can test more water in the long run and is more accurate. It's important for you to understand your water values to see how this many fish will impact a tank so small. If you have the test kit, could you post us the values for all of the above before a water change?

I doubt the aquarium salt is causing your issues. The issues that you're having are directly related to overstocking. If I were to be keeping all of the fish you mentioned in your tank (even though they are not compatible tank mates), I'd have them in a bare minimum 90 gallon tank.

Unfortunately, the only way to prevent more deaths is to rearrange the tanks/give away some fish. If this were my setup, I'd give the gibbi back to the pet store, use the current tank for the tropical fish (the neons and the glos) and get a 55g tank for the goldfish. If this isn't possible, I think you may have to rehome all the goldfish and the pleco.

You're in a tough situation and many of us have been there at one point or another. My first experience with goldfish was 5 common/commet goldfish and a common pleco in a 10g tank. I'm sorry to say that the fish that survived the ordeal (even after being moved to a 75g tank) died after a year. All of this waste that builds up in the water is tolerable for fish up until a period of time where some get sick and die. Those who don't get sick and die immediately are often stunted and will die later anyway.

I'm sorry