Hi,

1) I take it you're experiencing slow plant growth too? could be lack of nutrients, could be low oxygen - are any of your roots a tan/brown colour? Are the leaves perfectly green? How old are the plants?

2) Theres no reason, especially once the roots have grown into the spreader matt why you should not have the pump on 24 hours a day? Your water should be oxygenated enough, if not check the res temperature.

3) dont use canna, sorry