I used Rid-X in fish only tanks, and never had a problem.
It is sold a sort of dry flour-like mix, so soaking a small amount in some tank water in a separate jug or container first, will allow the bacteria to rehydrate (come out of dormancy) into the water, then pour the water infused the bacteria (as opposed to the cereal grit) will be much cleaner. For a 29 gal, you won't need much.
I used to soak it in filter socks, to keep the cereal from clouding the water, then after a day or so, rinse out the mix from the sock.
It took a week or so to see "gradual" results, it is "not" an immediate, or even over night, process. Just like cycling a tank, the bacteria need a bit of time to build up a sufficient population to overwhelm the cyanobacteria.
Especially with turtles, they produce copious amounts of waste, that help feed the Cyano.