The use of canisters does not mean Nitrates just appear out of nowhere.(Nitrate Fairy....maybe;D) Canisters are usually harder to clean then HOB's so people tend to neglect the maintenance on them.This is the only reason IMO a canister filter could be a problem in your system. Any unmaintained filter will become a nitrate factory including HOB'S. As long as you have ample LR in your system the loss of a small portion of bacteria should not be an isuue. Sorry to derail a bit. I use Chemi pure regularly. IMO its better to keep the water as clean as possible before a problem presents itself.Not after.