I was looking at a stocking calculator and they said that with filters the really volume they are suitable for is about 65% of the capacity stated, is this true? (It said even the well thought of brands do it), or do you guys use a different figure? I don't think I saw any reference to this here, but I often see people recommending double filtration (ie if a 100L tank add a 200L filter) but I thought that was just for goldies.
Just wondering.

Also, I was wondering what decides the filter capacity is it the filter rate, the size of the filter or sponge space? And if a air filter is used with biosponges, are there different sizes like the standard internals have? Or is it by air flow or sponge size? Just incase I ever decide to get a tropical tank, or a betta or paradise fish :)

I think filter capacity is decided by the volume it holds, Cause you can get bigger filters with slower flow rates for certain media types etc...

Though I may be talking absolute rubbish there

Just didn't want the post to get lost

filters are rated by the maximum size tank with low stock densities

going for one double sized is a good rule of thumb though

Thanks. Sorry about the badly written post, don't know how I didn't notice the typos at the time.
Anyone know how capacity works for the air/ sponge filter? Any recommendations for a good one?