A UV source will generally destroy most types of bacteria and will not selectively kill only bad bacteria. That said, good bacteria live in the bio-based material in the filter and will constantly be created. So, unlikely the UV light will be 100% effective in killing all of these that escape the filter; so some good bacteria should make it into the tank. In any case, since the filter media in the canister does not see the UV source, the good bacteria there is safe. As for a replacement capsule for the canister, not a bad idea to have a spare if one thinks they will break the existing one. I've had mine for tens years and never once considered that a concern. Changing the bulbs is necessary - the mfg likely offers a time period. Can't go wrong if one replaces every 6 months but again, check the manual for the unit.