Will APT FIX / APT FIXLITE kill my plants?Updated a month ago

If you dose within the recommended dosage? No.

The exception? Plants that are already very weak.
Both algae and the plants will be similarly affected in such a case.

Do not spot-dose APT FIX on Mosses, Liverworts (Riccardia and Riccia etc.) and Vallisneria (Tape Grass) as their cell structures are much more similar to algae than other plants and are therefore more susceptible to being affected. Use APT FIXLITE instead. It is gentler than APT FIX. 

However, already weak plants may still be affected, even with APT FIXLITE.Old leaves that are deteriorating are big algae magnets. Plants abandon old leaves and focus on defending new growth. Old leaves have limited capacity to recover once they start to deteriorate, and it is better to cut off these old leaves and allow the plant to channel energy to new growth. It is not worth treating all the deteriorating growth with algicide as deteriorating leaves will not recover in the long run even if the surface algae was removed.