I have been using those Jobe plant spikes in my aquarium since I started. The ones like these Amazon.com: Jobe's 5001T Houseplant Indoor Fertilizer Food Spikes, 50 Pack: Patio, Lawn & Garden My root feeders have been doing great. I had one sword plant outgrow my 29 gallon tank using them. I have fish, snails, and shrimp and have had no problems. I have even had the fish dig them up and take a bite and none died. Though I would not recommend that.
What is your stock of fish? Could I use fert sticks with rams???