Could be a nitrate spike. The otos probably died, ammonia and nitrite spiked, and then the nitrate could have spiked. Excess nitrate=brown algae, AKA diatoms.
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current setup:
15 gallon, planted w/ pressurized co2, 55watts PC lighting, EI fert dosing.
-5 harlequin rasboras
-7 Aspidoras pauciradiatus (sixray or false corydoras)
for reference: my name is Julie 
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