I had the same thing happen one time. Ended up being and algae bloom. Not saying this is happening to you but more than likely it is.
Mine took almost to months to go away. Reg water changes didn't make a difference either. It seemed to make it worse.
If you are not using filtered water, that can add to the problem.
I agree with adding in more ceramics to your filter. You can also use bio-balls in the Fx5. I do and they work just as good as the ceramics. Just don't put them on the top tray. Only the middle and bottom. Id also maybe up your filtration with another Fx5. Always better to over filter.
When cleaning out your filters, don't do them at the same time. Just clean one, then leave the other alone. Then the next month, do the other filter. Cleaning them both at the same time kills all your good bacteria. I run 3 Fx5's on my 360 gallon. I have a high bio load and believe in over filtering.
But I only clean one filter one month, second the next month and third the 3rd month.. This makes sure I don't kill off the good bacteria in the filter system.
Also, If Im right, you have a drip system to change your water as well as do reg water changes? If that is the case, you are def not letting your good bacteria settle in. So Id cut back on your water changes if you are also doing a drip system. Just do it once a month. Let your drip system do the work.
Maybe even cut back on your lighting schedule also.