if you want to know a little more about that type of filtration try this may help

i think the water fall filter would be best.

They are the easiest to clean and you want to keep your tank clean

They both have good points and bad points. A water fall filter does a better job of adding oxygen to the tank, but a submersible filter can usually clean out the corners better.
You are probably having your fish get sick because of a lack of beneficial bacteria. This bacterial cycle turns fish waste (ammonia) into nitrites, and then converts the nitrites into less harmful nitrates. You change part of the water weekly to keep the nitrates from reaching toxic levels. And the carbon in your filter removes many of the other toxins.
When maintaining your tank, you have to be careful not to stress the bacteria by using water with no chlorine and the same temperature as the tank. If you clean out too much of the tank, the bacteria starve to death before your fish start pooping again.
I don’t feed my fish for two days after a water or filter change, so the bacteria has a chance to recuperate.
When you set up a new tank, it takes time for the bacteria to develop. Ask the pet store for a handful of gravel (keep it wet) from one of their tanks to seed your new tank. You can put it in a mesh bag (even a stocking leg) if you don’t want it there permanently, but leave it in for a week. Only add one fish every four days or so, and don’t feed the fish for two days after you put it in (fish can go a week without eating).
A

A water fall filter is generally a term used for a type of pond filter. It’s not uncommon for ponds with Koi, and/or goldfish. The 2 major types of filter in freshwater tanks are under gravel (UGF), and hang on tank power filters (aka HOT). In general UGF have great biological filtration, but poor mechanical, and chemical. HOT filter are the reverse with great mechanical, and chemical, and poor biological. In general I recommend a HOT filter with a dry/wet wheel (aka bio wheel),or multiple filters. (Ideally with 2 filters one is changed regularly to keep the carbon filter fresh, and the other stays all the time. )
PS- There are also Cannister Filters, and Corner Filters. Corner filters might be an okay choice for under 10 gallon tanks. cannister filters are pricey, and are generally for 60+ gallon tanks.

i would not use an under gravel filter they are to difficult to keep
clean as they suck all waste under the gravel the outside filter sucks the water through filters which take the waste out of the water,returns clean water to the tank which makes it the best one to use . . Has to be big enough to clean all the water in the tank in about 2 hours. Hope this helps