In short Reverse Osmosis De-ionization units remove all dissolved solids from your water. It basically takes in your tap water and filters it through sediment and carbon then the RO filter lets clean water pass through it and it "rejects' water that won't pass muster (which is waste water and you send down the drain or use for plants or whatever, it's like the water you'd get if you used a Brita) and the DI stage removes what little has slipped past the RO filter. Treating tap water doesn't do that, it leaves everything in there and all it does is neutralize some chemicals but it doesn't remove organics, heavy metals, and other chemicals, which is what is going to feed algae you don't want. RO/DI units will definitely cover their investment cost quickly especially if you're buying water. You can find them between 100-150 bucks and will provide you 1,000s of gallons of water before you need to replace filters.