It depends on the situation. If you're changing the water it is a good idea to use a water conditioner to help maintain it biologically and chemically. Or if you are handling fish using a fish net. If that is the case then use something like stress coat for handling fish to help build up their natural slime coat.
55 Gallon-two clown knifefish, chocolate pleco
46 Gallon Bow Front-Planted-swords, java fern and African cichlids, sail fin pleco
29 Gallon-black ghost knifefish, bristlenose pleco
10 gallon-brackish, figure-eight puffer, knight goby