What's causing my diatoms then? My tank has been going over a year with pool filter sand and I still have to wipe down sides of tank from time to time
My understanding is no tank will ever be 100% algae free. Without a cleaning crew and left untouched, almost all tanks will show some type of algae development over time. I've heard people say "an algae free tank is only possible with no lights and no water".
Causes for yours could be several; but the most common are elevated nitrates, lights on too long (or ambient light from a window), bulbs need replacing, excess nutrients...
I had brown algae off an on my tank for about 8 months. First I changed the bulbs and lowered my light schedule down to 10 hours and it seemed to lessen. I also noticed a window was letting light into the tank early morning, basically meaning the tank was still lit for over 12 hours a day. I just make sure to keep the blinds closed on that window now.Then I started feeding less and added a few more plants, and it lessened more. Last I started dosing small amounts of fertilizer and it was hardly noticeable.
I finally added a cleanup crew and never saw brown algae again. For me it was a few oto's, MTS, and a couple zebra nerite snails. I add some blanched cucumber once a week to supplement just in case
That's not to say I have been algae free (BGA fight recently, my fault), but the battles are fewer and further between now. Just have to try and keep things balanced, every tank is different.