start with RO/DI
like this one from bulkreefsupply. Nitrates are not the only thing in tap water you need to keep out, also need to keep out phosphates, TDS
. I have Nitrates at 160ppm. They stay at 5ppm and I am trying to get them down to 0. Its possible you released nitrates from your sand bed when you cleaned it.
canister filters can become nitrate factories if not cleaned weakly. the one I had was more problems that it was worth. Phosban Reactors I think are better for media and easier to clean.