What's the best type of filter for a small tank?

External filter

advantages - the filter is outside the tank so there's more room inside the tank. Too, there's more room for media. It's easy to maintain. Super filtration IMO, compared to internals, UGFs and sponges.

Disadvantages - may not fit on the tank. The tops can rattle (an easy fix), and some can be a little noisy (fixable as well). You cannot put a tank flush against the wall because they hang off the back. You can still get it pretty close though. If you let the water drop too low the filter may not restart after a power outage. The low water level can cause a backwards siphon, draining the filter, which can prevent it from restarting. However, as long as the water level is not allowed to get too low, the filter will restart fine. Some brands can prime themselves, others can't.

Internal filter

Advantages - often quiet, because the motor is inside the tank. Easy to fit into smaller tanks. Will always restart after a power outage.

Disadvantages - takes up space in the tank and can detract from the beauty of the tank. The quality of a lot of small internals just isn't that good. They have limited space for media. Some require an air pump. Some are a pain to maintain. Too, while it might be quieter for us to have the filter motor in the tank, it's probably not quieter for the fish.


Someone else can talk about UGFs and sponges.

Sponge filters:

Pros

• Take up less room in a tank than internal filters
• Are a simple design so not much to go wrong with them
• Provide some mechanical and some biological filtration
• Very inexpensive
• Simple maintenance - squeeze and go
• Provide very little current/water disturbance
• Very simple to make yourself if so inclined

Cons

• They require you to run an external air pump - noise can be an issue
• They typically don't provide a lot of flexibility - nowhere to add carbon if needed, can't be expanded with filter floss or other media
• They do take up some room in the tank
• They are kind of ugly

I don't have anything to say about undergravel filters, as I have never used one.


I switched from sponge filters to in-tank filters in my 5 and 10 gallon tanks because I couldn't deal with the noise of the air pumps. There is a small amount of splashing sound with the in-tank filter as evaporation occurs, but it wasn't as noticeable or as bothersome to me as the air pump rattle. That's honestly the only reason I switched. The sponge filters did a great job as filters. I also like the ability to put any kind of media I want into the internal filters.