This might be a stupid question,Modern HOBs are really good, just needs to be sized right for the tank. I've always gone for externals though to keep equipment out of the tank and and you typically get more powerful filters with more turn over in externals which is important for messy fish like cichlids.
Wills
Usually you need one filter per tank as otherwise you would have a very elaborate system of pumps etc. 2 exceptions to that though, if you have air driven sponge filters you can use a single air pump to power them all or if you have a sump you can have a rack of tanks (2-3 on top of each other) and you can have them all connected but I dont know how this works lol.This might be a stupid question,
But if you have 2 or 3 tanks do you have separate filters for each?
Or is there a filter that can do more then 1 tank
Not only are they not needed, they are outdated, messy, and a pain to deal with.Are under gravel filters needed?