I shouldn’t think they will harm him, it must happen in standing water in the wild. I use these in my wildlife ponds and my dogs have drunk from them for 3 years now without ill effects so maybe worth thinking about. If the larvae are hatching, maybe you are leaving it too long between water changes? twice daily should guarantee no eggs will hatch in the water.
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The dunks are more used in the US, can I suggest you find a US lizard forum and ask about their safety on there before trying them. I don’t use them in my tortoises water.