Sure! What kind of fish do you have in those tanks?
For your 2 gallon, this red sea deco art nano filter (hangs over the back) would probably be great: http://www.petco.com/Shop/Product.as...amilyID=12148&
It's great in my 3 gallon with a female betta. Hagen Elite Mini is another filter of about the same power, but you put it inside the tank:
Both have adjustable flow rates.
There are more choices out there for 6 gallons. Depending on the the kinds and amount of fish, look for a filter with a gph (Gallons Filtered per Hour) of 40-90. If its in the upper range, an adjustable flow is very useful.
With all filters, do not toss out and replace the cartridge or sponge like the box instructs. This will disrupt your nitrogen cycle! Its best to keep them for a long time and safely clean them by swishing in dechlorinated tap water (or in a bucket of dirty tank water). Small tanks are more difficult to keep cycled and steady, so buy some bio-max to stuff in the filters for biological filtration. Never clean the bio-max, and do small frequent waterchanges to keep your tank healthy.
Filters are necessary for almost all tanks. Make sure to read up on the nitrogen cycle if you haven't already