I'll start with my tank size and the fish in it. 36 gallon with 8 harlequin rasbora, and 6 black khuli loaches. Filter Fluval C3 which is rated for 20-50 gallons. I've had the tank up for a while but just recently added the loaches.

I just noticed tonight that one harlequin rasbora has his mouth to the surface. There's only 1 fish doing this and the rest of the rasbora and loaches are swimming around fine. Could this be a lack of oxygen issue? Though I would think it would affect more then 1 fish or is this just a sick fish?

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36 gallon - 6 Rummynose Tetras, 6 black khuli loaches
20 gallon - 4 peppered cories, 5 danios, 4 glowlight tetras
10 gallon - cherry shrimp, jungle val, Red Ludwigia, dirted

It could be sick, if its a lack of oxygen, make sure your water isnt too hot and that you get enough surface agitation. The warmer the water, the less oxygen there is. What are your water parameters?

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Frontosa Fanatic

He didn't make it through the night. I found him stuck to the intake tub on the filter :( Water tests turned out normal. Could he just have been over stressed from the new loaches? They were added last thursday.

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36 gallon - 6 Rummynose Tetras, 6 black khuli loaches
20 gallon - 4 peppered cories, 5 danios, 4 glowlight tetras
10 gallon - cherry shrimp, jungle val, Red Ludwigia, dirted

possibly, but he could have just had a weakened immune system from the stress of adding the loaches, sorry for your loss :(

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Frontosa Fanatic