03-09-2011, 01:16 PM
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Correct, science has now discovered, or at least is leaning to thinking, that Plecostomus species do not actually digest the wood. It seems to be taken in with the algae they graze from it, but it passes through without any "nutrients" being taken from it. Sort of like roughage I suppose. I posted a link to this study a few weeks back in the Tropical Freshwater Aquarium Fish section if you want to track it down. It was a study mentioned in Practical Fishkeeping.
To your question on otos and such: a BN will handle any common green algae or diatoms in a 20g. There will be none for other fish like otos unless you have an imbalance (too much light). Otos are more likely to last if they have algae when first introduced to a tank, as it is their natural food. Later they will easily adapt to sinking foods, but initially it is best to ensure algae is present. Same holds for sensitive fish like Farlowella. And a BN would obviously be happier too with algae at first. If you like the otos and want them solely as "fish", fine; but if it is only for the algae-eating aspect, your BN will handle that on its own.
And remember, all of these "algae eating" fish do not eat any and all algae, only specific types. Most the common green and diatoms. A few specific species will eat brush, etc., but not the fish we are discussing here.