Originally Posted by
SantaMonica
Several smaller bulbs will work fine.
Bald spots are caused by too much light relative to how much iron the algae is getting. The more light that algae gets, the more iron it needs too. CFL bulbs tend to make bald spots because all the light is concentrated in one spot, while the edges of the screen don't get enough light. Aside from switching to T5HO bulbs, here are your options from best to worst:
1. Get more flow. This will deliver more iron to the algae and let it grow again in the bald spot. Your screen needs to be very rough, however, to hold on to the algae in the higher flow.
2. Add iron, such as Kent's Iron+Manganese, per the instructions.
3. If adding iron did not fix it, and you can't add flow, then move the lights farther back. This will spread the light out more evenly across the screen.
4. If you can't move the lights back, then either use smaller bulbs, or reduce the number of hours that the lights are on.