Keep in mind that RO units need upkeep too. You need to change the sediment and carbon filters every 4-6mo. The DI cartridge usually has a color change, mine last about 6mo. The membrane should last 2years but if you do not change the other filters when they need it, it will shorten the life of the membrane. You also have to be sure you run water through your unit at least once a week or bacteria will grow in the sediment filter (had this happen). I just thought you should be aware that all this costs money too. With such a small amount of water you will use with a 25gal tank, I would just buy it, but it is your choice, and I would test it occasionally you may even want to ask the LFS how often they test for TDS and change their filters. If you are planning to upgrade your tank to a bigger tank you may want to go ahead and buy an RO unit, but with a 25gal it would proabably be easier just to buy the water.