Which is better for a tropical aquarium, an external filter system, or an internal filter system?
Definately an external filter. This is because they have a large area of biomedia (often ceramic media such as white ‘tubes’ called biomax or similar) which is where the beneficial bacteria can live and function efficiently. Most internal filters have just a sponge, which only provides ‘mechanical’ filtration, ie all the lumps of muck get stuck in the sponge. Trouble with this is that any bacteria in the filter have only got the sponge to live on and when this gets all clogged up with muck (in about 5-7 days) they cannot get enough oxygen to work correctly. This is why you should clean an internal filter about once a week (and only clean the sponge in dirty tank water and not under the tap). In an external filter, you still get sponges which collect most of the large lumps of muck, but you also get other types of filter media such as chemical and biological to provide more efficient filtration. You often get carbon in the filter, but I would replace this with extra biomedia. This is be
