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Nitrate build up over time if you feed very little. Most people feed more than the fish need, it is why nitrate builds up fast enough to require more than "once 2 months" partial water change. You must be feeding very carefully to achieve low nitrate concentration even 2 months without water change. Algae has nothing to do with it. I have a tank full of plants which absorb way more nitrate than algae will ever do, my nitrate will build up faster than your tank if I don't do water change frequently.
Water change is also for more than just to remove nitrate. The process of producing nitrate is acidic. Over time, the water buffer will be gone and the PH can crash suddenly. Fish don't like sudden change. Sudden crash of PH can cause problem, and can cause problem again when you finally do a water change which can restore the PH (suddenly) again.
There is also other dissolved waste other than nitrate. Fish are basically swimming in their own waste if you do not do partial water change regularly. It is not healthy for them in long term.
Oh, how long have you had that common plecostomus? When they are adult, they barely eat any algae at all. They produce a lot of ammonia.
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