The 18-gallon tank is now in the diatom stage, which precedes the green algae stage. At this point, I no longer need to feed the opae’ula. The diatoms are an ugly brown algae that look more like dirty smudges. Fortunately, the opae’ula will feed on them until they’re replaced by nice green algae that’s more permanent and attractive. You don’t need to clean the diatoms from the glass since the opae will do that for you. It’s amazing how much diatom will grow and how the tiny opae are able to consume them all.