You might try putting in a daphnia culture, your tank sounds perfect
for them. You might need to set up a refugium for them so they don't
all get eaten. Okay, probably not a serious solution, but I think it
could be a cool set up anyway.

How is your UV sterilizer set up? What is the wattage of the bulb? (sb
13wt) What are you using for a water jacket? What is the flow rate? UV does not penetrate very far into the water, and the algae will make that worse, so .5-1cm tops and the longer the water takes to exit the better. Jebo makes a fairly good sterilizer that you can get on eBay for around $50.