RO/DI water isn't necessarily essential, but success starts out with the water you're using. RO/DI is pure water, no phosphates or silicates or any other mysterious dissolved solids that would be in tap water. RO water has a much lower TDS than tap water, but still can cause algae problems and other issues. For a beginner, RO water/distilled water is fine, but I recommend a RO/DI system if you can afford it. If not, get RO water at the very least. I've never seen that tap water filter used or reviewed by a saltwater hobbyist. There's used RO/DI systems you could get or some RO systems for $80. You want the lowest TDS reading possible.
THe link to the filter says only treats 50-150 gallons which is outrageous. Even for a nano SW tank, that cartridge would have to be thrown away in a month or two. Not worth the money.
