Hi Reino and welcome to WAB. Re frog spotting, after dark is best, with a torch. You may find you hear plops or splashes before you make it to the pond's edge, so all you may see is disturbed water. If you had frogs last summer, there's a good chance they'll be back. To be safe I wouldn't suggest significant intervention wrt weed removal etc just now, but not really sure how much disturbance would dampen their ardour
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Good luck
M