Just keep wiping them off. I know its a pain (I've had diatoms last up to 8 months) but it's not worth screwing up the cycle by adding fish, especially sensitive ones such as otos. They'll go away eventually.
It may not be as obvious that silicon can be employed in the metabolic processes of diatoms as well. Without silicate, diatom cells cannot divide, and its capacity to perform energy producing processes (photosynthesis and glycolysis) are diminished. In addition, synthesis of DNA, protein, and chlorophyll halts. These effects may be a result of silicate's role in the regulation of some enzymes.
Analysis finds that primary production in the model is ultimately controlled by phosphate, with silicic acid abundance controlling the fraction of the total produced by diatoms.
I found this link:
http://www.mbari.org/staff/conn/botany/diatoms/jennifer/intro.htm
Facinating little organism, a diatom. So getting rid of orthosilicic acid (Si(OH)4) could be a key to keeping them from growing.
from: Role of diatoms in regulating the ocean's silicon cycle by Andrew Yool
So perhaps my idea of using some plants to absorb the phosphate might have some merit.
So is this the end of my cycle? For the last month I have seen low readings (but not zero) of all three parameters while I was increasing the stocking from the original 2 platies.