f your taking about a filter, then id get external cannister filter, much better than an internal and doesnt require as much cleaning. might seem a bit pricey, but definetly worth it.

 Cant help if its salt but if its tropical, before you do anything I would have the water tested. If the water is lacking something or it has too much of one thing, you can get a different additives for the water and its amazing how much that can clear it up.

I would take a sample to an aquarium shop and get them to test it and see what they think which is the best way to go.

cheers

That can add tannins and stain to the water making it from yellow to brown

External Enheim pump and cannister filter - yes they are expensive but really are brilliant and when you get everything in balance only need to change filter media once every three months 

As yes PH is critical as is dechlorinating fresh water when you do partial changes.    And dont do what we used to and collect rainwater as, even in Perth, it is slightly alkali (probably the result of our proximity to the sea)

I'd suggest the cheapest most reliable filter is to get an internal pump prob 1000 l/hour. plumb to a box (25 litre plastic tub) filled with media like 'bio-balls and Japanese matting that sits atop the tank. Plumb a line back to tank via gravity feed. You can build an overflow in to the box incase usual return gets blocked.

Pump could be around $50
Box, media, plumbing maybe another $50.
Can be obtained 2nd hand for cheaper.

But if you do have $150. The external cannister filters are good value and work well.

Also check aquarium forums. You will find info that explains that these filters are biological, rather than mechanical filtration and require continuous water flow. Can be upto 6 months before the flora of bacteria reach the potential of 'cleaning' the water.

The easiest way to maintain an aquarium is to make certain the filtration is more than sufficient.

Just to add more info.
Sea water or fresh water the same filtration process is required. Maybe slightly different spp. Of bacteria colonize the media.

Also better to just use the water as it is from the source. I.e tap water use from the same source as consistency is easier (cheaper) on yourself and the fish, rather than trying to balance the pH.
Sea water - try to minimize collecting contaminated sea water as in from boat ramps (motor oils).

Tap water pH, hardness etc will be ok for most freshwater fishes.

99% of products at aquarium shops are not necessary.

I have a tropical tank, Ph levels are fine water clarity are fine but could be better.

When i got the tank it was a cheap 30dollar internal pump and I'm looking to upgrade.

I'm looking at getting more air into the tank as well, thanks.

Most of the guys covered it already. If you're looking for clearer water you most likely need a filter (with a fine mat section) as a pump will just push things around without doing much (and maybe stir more debris up). Other possibility is algae/bacterial blooms which can cloud the water.

If you're looking for a cannister filter (seriously the best type unless you build a full sump) then Vebas have the 2213's on sale atm which would do the job perfect. There are cheaper options, but the eheim ones are the best quality, easy to clean (just open it up and rinse the media with dechlorinated water) and do the best job (better than most other filter types you can buy). Otherwise, look at hang on back filters (as long as its a reasonable one). Dont ever bother with internal filters, they're just crap. A cannister will last you years, provide much better filtration, and much better habitat for the beneficial bacteria you need.  If you're really cheap, then get some fine white matting (couple of bucks) and put it over the pump output with a lacky band but expect to replace it often.

Definitely dont bother with a gravel filter! They are seriously the worst kind and can do more harm than good. There is no need to 'get air' into the water. Air in the water doesnt do much (and clouds it up if its fine, but rarely in FW tanks). The big area of water you have at the top of your tank absorbs much more oxygen than a few bubbles. If you need more oxygen, then a sump/biofilter would do it. Realistically, as long as you have circulation (pump or filter) and the fish have room to swim, and the tanks not overheating then they'll have enough oxygen.

Btw, what fish do you have?

what type of set up have you got? (freshwater, salt or brackish) also what do you have in the tank?

Thanks, went out and got a 200$ 4 stage cannister filter aswell as a air pump, already the water is looking crystal clear. Getting some air in the tank is good as well(fish are loving it, swimming through) Got my water tested again, perfect so thats good.

I have some silver sharks, guppies, mollies, rainbow sharks, bottom cleaners and black and red sharks(catfish).

Cheers again guys, appreciated.