This will not hurt nematodes. They can both be used together to break the larve cycle of fungus gnats.
Products such has Gnatrol/Mosquito Bits/Dunks and MICROBE-LIFT/BMC have the active component BTi. (
Bacillus thuringiensis subspecies
israelensis (Bti).
This is only harmful to mosquito and fungus gnat larvae.
I recommend MICROBE-LIFT/BMC because its more concentrated and liquid form. Mosquito dunks and bits are not as concentrated. With dunks and bits you typically want to use a strainer bag when soaking them in your water (added step) vs. MICROBE-LIFT/BMC you add two drops per 5gal of water for a more potent and easier mix.
@McShnutz MICROBE-LIFT/BMC is water soluble and safe for irrigation and hydroponics.