There are a couple of possibilities:
-Nualgi may make the problem worse by causing growth of more Diatoms.
-When Nualgi is used better species of Diatoms will grow and these will out compete the present species growing in your tank. The new species will be better food for fish, so they will be consumed. In tanks in which Nualgi is being used, there are no complaints of excess diatoms, since the diatoms are being consumed. As you are having a problem with accumulation of diatoms, so it is possible that the diatoms growing in your tank are not being consumed, as they may be inferior species.
– It is possible that the brown algae growing in your tank is not diatoms at all but some other algae that is also brown. If so, Nualgi will help