Basic guidline is (1) Root Tab per 4x4" area, buried at least 2" down. This is just a suggestion, and should be adjusted according to how many plants you have. More plants require more fertilizers, but its best to start slow and work your way up if you're not familiar with these.
Our root tabs are made with Osmocote Plus plant fertilizer, which has 11 essential nutrients and is fish and shrimp safe. This is an all-in-one fert for root feeding plants. These should be good for 2-6 months depending on plant uptake.
