Sponges. Sponges are essential aquarium tools. For years their only pitfall lay in the additives that keep cellulose sponges soft and squeezably marketable inside their plastic wrapping. I have poisoned fish with a new sponge that hadn't been squeezed out thoroughly in water.
Now the US Environmental Protection Agency has registered antibacterials variously named "Ultra-fresh" or "Stay Fresh" or somesuch, which are being incorporated into the very cellulose of sponges, to keep bacteria from establishing in them. These sponges are labeled "not for aquarium use" but it's worth repeating here that they will release toxins.
Manufacturers' websites. Nowadays almost all of the major manufacturers of aquarium equipment have their own websites: some are listed in the "Links" page. When I was given a gently-used Fluval 203 canister filter, for example, I was very relieved to find the instruction manual for it available on-line at hagen.com .