Since your tap water is high in nitrates and causing algae I would also consider ammonia or ammonium contamination. Even at very low levels NH4 will cause algae blooms. The test kits used by most hobbyist are notoriously inaccurate. So not seeing any NH4 does not mean it's not there.
With nitrate levels that high I would be more concerned about the health risks of drinking it specifically infants. The EPA limit for nitrate is 10 mg/L which is essentially the same as ppm. As I said before test kits are notorious for being inaccurate so you may want to consider sending a sample to your local extension office. If you're on a municipal system you should be able to get an annual quality test. The suggestion of an RO unit is IMO the best advise I've seen. Below are a few links to articles, including EPA, about nitrates in drinking water.
Nitrate in Drinking Water
Nitrate and Nitrite in Drinking Water
Basic Information about Nitrate in Drinking Water
Thanks for your concern. The tap is 45 ppm, according to our water superintendent. He told me to call the county health dept. and they would explain to me why it is okay. I never did. As far at the EPA limit goes, it seems the only thing that must be done is to increase frequency of testing. No changes have to be made, I was told. I will read your links.
Yeah, the RO is looking like the better remedy.