I'm in westchester and we have the same resevoirs so its pretty much the same water as nyc. When i first set up my tank the diatoms were a mess for about 2 weeks. It was almost to the point where i thought I bit off more than I could chew with a hi tec planted tank and was about to scrap the tank. A friend of mine interceded and told me to start adding potassium and to double the amount of excel I was using. Literally the next day all of the diatom was gone... I mean 100% like it never existed, completely gone. Unfortunately I changed more than one thing so I cant point to any one and say it fixed it.
I added ottos and japonica to the tank and although they did eat the diatoms they were not solving the problem. At its peak, the entire tank was covered. It was a complete mess, and I didnt think it was ever going to get better.
NYC and the surrounding suburbs has some of the best tap water in the country and although there are silicates in it, its great water. I'm a little obsessive when it comes to water changes (atleast 5 gallons a day in a 65 gallon tank). I use tap with some conditioner and 1/2 teaspoon of discus buffer and the plants and fish seem to love it. My plants actually turn more red right after major water changes which makes me think theres something good in the tap water that Im not adding otherwise.
If you hang tight the diatom will disappear, even if its so bad you think there is no way its ever going to clear up.
Good luck and hang in there!