I had leeches in my one tank before....the thought of them makes my skin crawl!! They really are a nightmare!!!! I am guessing you have some type of filtration system. Inspect the filter housing and filter media for any eggs. Granted, with a pond, they can lay eggs anywhere. Mine seemed to prefer my filters/media. There are chemical means for treating them as well as natural approaches (leech traps) but they really are tough to get rid of because all you need is single egg to start a new population. And the eggs can basically survive anything (heat, cold, drying, chemicals,etc) and only hatch when conditions are right. That's the bad news. If you can, inspect your fish for leeches, bite marks or missing scales.
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