The short answer
Pinholes and small holes that appear in the middle of leaves — especially older ones — are the classic sign of a potassium (K) deficiency. Plants pull mobile nutrients like potassium out of old leaves to feed new growth, and the old tissue breaks down where it’s been drained. The fix is to dose an all-in-one fertiliser that includes potassium.
Deficiency, not pests
It’s tempting to blame snails or fish, but true grazing damage looks like ragged edges and missing chunks, not neat pinholes forming inside intact leaves. A nutrient-starved plant also grows thin, brittle leaves that then tear easily, so the two problems often go together. Grow healthier leaves and the holes stop appearing.
Potassium is the usual suspect, but a broader shortage of nitrogen, phosphorus or trace elements produces the same tired, deteriorating older leaves. Rather than chasing single elements, a complete all-in-one liquid fertiliser covers the lot. See our fertiliser picks and the plant fertiliser hub.
Feed roots as well as water
Heavy root-feeders such as Amazon sword and Cryptocoryne wendtii take a lot of their food through the substrate. If they’re the ones with holes, add root tabs near their bases alongside your water-column dosing — the liquid vs root tabs comparison explains when each matters.
Give it time
Once you start dosing, don’t expect the damaged leaves to heal — they won’t. Judge success by the new leaves growing in clean and whole over the following weeks. If new growth is still holey after a month of consistent feeding, step up the dose or check that your light isn’t outrunning your nutrients. Related reading: why leaves turn brown and why plants turn yellow.