well if you had a bad alge problem you may have root rot check the roots, is there slimy brown stuff on them if so that is your problem some h202 will help with that next time as a preventative measure use hydroguard its a biological water treatment that stops alge growth but do not use it in conjunction with the h202 as they will conter act each other making them useless. do an application of the h202 then wait 24 hours and add hydroguard.