If your tank is heavily planted I would leave it on on a night plants take in oxygen at night and produce it during day so if your tank has lots of plants they could take in the majority of the oxygen at night and not leave enough for the fish
sorry can't help you ID but interesting read hope you get to the bottom on it. Wouldn't think it was flukes tho I've had them in my koi pond and wow you know you have them I lost a lot of fish due to secondary infection.
all my fish love cucumber everything in my community tank and my Malawi tank. 1/4 of a large cucumber lasts about 30 min In the Malawi tank.skin the lot gone lol