test your water and see where all the params are. At this moment, focus on phosphate, nitrate and ammonia. If there is ammonia in the tank, then theres a source feeding nitrite, which will feed nitrate. Phosphates will increase as well, and also the more you feed. Something that will take care of the algae problem is using RO water and 20% water changes once weekly until problem is gone.
Watch your feeding and what not, overfeeding will cause this problem quickly
What are you feeding your two mantis' ?? and how often do you clean??