Why memory cannot answer this question
Every keeper with a pleco eventually wonders whether it is doing anything, and memory is a hopeless judge: algae grows slowly, you see the tank daily, and the fish is conveniently nocturnal, so it always looks like a pet and never like an employee. A log answers it because the comparison is written down. The trick is to log the right things — not just that the pleco exists, but what it eats, what the algae is doing, and what else changed in the tank at the same time, since light hours, feeding, and a new plant fertiliser all move algae more than any single fish does.
Set up the stock side of the log
Give each algae eater its own running entry in your maintenance log, alongside the water tests you already record. Note the species as precisely as you can — a bristlenose and a common pleco are both sold as plecos and have utterly different futures — plus the date it went in, its rough size at arrival, and its size now, updated whenever you notice growth. Record what it is actually eating: still rasping the glass and wood, or first in the queue for wafers and the other fish's food. A growing pleco that has switched to prepared food has quietly retired from algae work, and the log is where that career change becomes visible.
Score the algae, simply and honestly
Against the stock entries, keep a weekly algae note in fixed terms so weeks can be compared. Walk the same short list each time: film on the front glass, growth on the wood and rocks, tufts on plant leaves, and strands in the moss or carpet. Score each as worse, unchanged, or better than last week — three honest words beat a paragraph of impressions. Take one phone photograph from the same position, with the same light, at the same visit. Photographs are the part of the log that cannot flatter you, and side by side they show a trend your eyes, adjusted daily, will have missed entirely.
Read the log like a decision, not a diary
After four to six weeks the entries should let you say which of three situations you are in. If the algae scores are steady or improving and the pleco still grazes surfaces, it is helping, and the job is to keep its conditions the same. If algae is worsening while the log shows the pleco queuing for wafers, the fish has outgrown the work or out-eaten the supply, and the fix is feeding and light changes, not a second, bigger pleco. If algae worsens alongside longer light hours or heavier feeding in the notes, the pleco was never the lever at all. Watch the common pleco case especially: a log showing rapid growth is your early warning that this fish will need a far larger tank, whatever it does for algae.
Keep the log with the fish's records
An algae log only works if it survives between water changes, so file it with the tank's test results rather than on loose paper. Stored in the pleco's KinsLe Pet Passport, the stock entries, weekly scores, and photographs sit alongside its other records, ready to show a shop or a fellow keeper exactly what the fish has been doing.