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Depending on the size tank you have would reflect whether you should get your own ro/di unit or buy it from the pet store. You can get a water making machine for as low as $65. The good thing is you don't have to lug buckets of water back and forth either.
Another thing is when you get water from the lfs you need to test the salinity yourself just to be on the safe side. For me it is more cost effective to make my own water.