DO NOT waste your money on a "tap water filter" -- 99% positive that will only be a carbon reactive filtering unit used to make tap water "taste clean". It'll last two weeks before you have to replace the carbon.
You can find a 4-6 stage RO
unit on eBay that'll do the job just perfectly. Just make sure they supply an appropriate website to purchase replacement filters from (maybe be a good deal but if you can't replace the RO
piece its worthless after 6 months -- or less depending on how frequently you're using it).
Typically these will run you $100-150. This is what my step-father got.
I personally drop the $3.50 on two 10g bottles of 5-stage filtered water from the machine in front of the local HEB (grocery store) every 10-15 days (~$300/yr. thus proving its cheaper to buy an RO
unit with $45 in replacement parts every 6-9 months -- I can't get an RO
unit as my apartment complex would kick my butt if I broke any of the plumbing; pay enough in insurance for the fish tank