I got this chamber used from another reefer (John). The external AA refugium drain is quite noisy.
I played with a few configurations, and got it to be as quiet as possible.
I added an elbow with hole drilled on top to break any potential syphon. Also, the drain pipe outlet should not be submerged (but just above the water line). This will prevent the syphon and subsequent gurgling noises of the flush.
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I hope this helps anyone who buys an AA external refugium chamber in the future.
To be honest, I would not recommend anyone buying this AA external refugium chamber. The potential that something will get caught in the drain (like chaeto or snail) is too high and it will flood. There is no emergency drain, so it is a very poor design. Personally, I would not run this as a refugium as the risk to flooding is too high. I would need to drill another hole as emergency drain or build a weir if I would run this as refugium.
For now, I will just run this empty. No plans for now, maybe a very deep sand bed, miracle mud or allow bacteria to grow. All depends on how my nitrate looks when the tank settles down.