CR, for ur setup to use canister need to place the pump into the FGT. Use a less powerful pump if worried of flow too strong.

Instead of 100% water change, why not try first 80%. But maintain ur current setup, since u hv nice wall algae + some green water. Then mayb ur gfs not affected too much.

Do u check ur water hardness after changing water. Mayb KH/PH too low. I always add KHbuffer to maintain KH lvl of 3deg.