Please Help: My plants stopped growing

[VB123]

I have a ten gallon freshwater aquarium (see picture); I've kept it running for about 4 months now, the water levels are good; I do 30% water changes once a month or so with reverse osmosis water, dose with Aqueon Plant Food once every 2 weeks, and feed my fish, a young angelfish and bumblebee goby, 5/7 days a week. It's overrun with plants now, but it seems growing has screeched to a halt, or at least a crawl. Before, new shoots were coming up all over the place, and the mother plants were getting bigger, but for the past two weeks it's been a standstill. I'm wondering, do I not have enough fish/waste in the water for them to grow on? Would adding another fish or two help? And what kin? I have bad luck with adding new fish to established aquariums; something always goes wrong. Also my tank is dirted and cleaned by two in-tank filters, but it's only biological and mechanical filtration! Much thnx

 

[tomojsg]

try dosing with seachem excel and flourish