You may have nutrient issues w/ the NSW
. Investing in a good test kit for at least PO4
would be a wise investment. 8-8.2 is a broad range for PH. PH will naturally rise and fall w/ the lighting schedule. If it's at 8 at the end of the lighting period, it may be dropping too low at night. Generally if it stays in between 7.8-8.6 with out too much daily fluctuation(<.2) your good.
alk - 240ish
I'm only familiar w/ units in meg/l or dkh
for those params. What units are the test using??