I recently bought a new 29 gallon aquarium for my 4 goldfish and my pleco. I have been using water to fill the aquarium that has no chlorine or salt, but it is leaving really bad mineral deposits on the glass and the plastic cover over the top of the aquarium. I was wondering if it would be safe to use tap water that doesn't have any chlorine in it, but it is softened. We soften it with softener salt, so the water would have some salt in it, but hopefully it wouldn't leave the mineral deposits. So my question is, would that small amount of salt harm my goldfish or pleco?

What is your current source of water for the tank? It must be hard water. Did you test the hardness? You can use your tap water if the water chemistry is okay. Check the tap water's pH, hardness, nitrate, phosphate, and perhaps other chemical levels. What is the source of your tap water (city, well)? We have well water. There are phosphate crystals where our water comes in to soften the water (replacing calcium, magnesium, etc. cations with sodium cations). This is super stupid since our well water is super soft but my mother insists it is not and wants it chemically softened. Despite going through the phosphate crystal "softener," I've always used our well water straight from the tap and haven't had problems with anything except super saturated carbon dioxide (which requires good aeration). Even though our water is soft, we can still get a film on various things. My aquariums aren't a good judge of that since I add salt to them and get salt deposits in the areas that splash water. It's good to add aquarium salt for goldfish and even the pleco. I use a tablespoon per 5 gallons normally. Goldfish are more tolerant of salt then plecos but both can take some salt. The little bit of sodium salt that may come from your water softener is not significant and shouldn't be a problem.

I must add that a 29 gallon tank is probably not going to be large enough in the future for 4 goldfish and/or the pleco (if it's a common variety) as they can all grow nearly a foot long when mature. Also, plecos have been known to suck on goldfish.