Well,I think my filter might be a little to weak.I bought the 10 galleon starter kit and use the filter it has.My tank is also overstocked,4 fishes in a 100 gall.,The water is always really cloudy.I do a 50% water change and ot looks a bit better.Then next morning its back to the same way it was.I'm thinking I might need another filter but I have a problem.That tank's hood only has room for 1 filter.Theres a small opening on the back that only fits a small filter of that size.I'm planning on getting an airstone to help with the water and oxygen to.So,So I need a new filter??

Use a hacksaw to enlarge the opening. Or run the second power filter on a different side of the tank.

Or failing to do that, go with an internal, canister, sponge or undergravel filter. Let's run through the options:

Internal

Pros: Not a big current. Fairly easy to set up and maintain.
Cons: Not a lot of media room which is what you need most now.

Canister

Pros: Loads of media space, good current and out of sight.
Cons: Expensive and probably the only ones that fit a small tank are Fluvals.

Sponge:

Pros: Easy to maintain, cheap to make (or buy), good biological filtration.
Cons: Might look unsightly in tank (unless you do some creative decorating), should be worked in tandem with another filter, sponges sometimes need replacing in which case you lose alot of bio-bugs/

Undergravel:

Pros: Biological filtration and mostly out of sight.
Cons: A bugger to maintain (unless you go reverse flow which shoots all your dirty stuff out from the gravel), should be worked in tandem with another filter.

Well,Thanks,those are good ideas
I need to think what type of filter I should get :/

since you are so over-stocked, I'd recomend something with a high GPH turnover rate.