Re-purposing aquarium heaters

Does anyone use aquarium heaters for various plant/tree things? I was thinking they might offer a lot of advantages.

Instead of using light bulbs under blankets to protect things from cold, for example, I was thinking an aquarium heater would all but eliminate any electrical fire hazard (which is particularly a concern with some marginally cold hardy things I have planted right next to buildings for protection), and it might come with a built in thermostat, too, plus the water would buffer temperature swings, avoiding any undesirable over-heating. I got the idea from someone from up north that posted something to a forum about growing zone 8 hardy citrus in zone 5. I was thinking it could also be useful in protecting cover-able things from late spring freezes, although I’d need a separate heater for each thing that was going to be covered (unless maybe they could be covered together.)

And it seems like it might work very well to heat an insulated cold frame like a large, re-purposed chest freezer, opened during the day and closed at night. A light bulb plugged into something like a thermocube might work just as well in that case, but I can’t seem to find anything like a thermocube that comes on around 65 degrees and shuts off at 70 (or thereabouts.)

I was thinking about how to possibly use an aquarium heater for a fairly large (at least relative to a standard heating mat for the same price), potentially very inexpensive source of bottom heat for rooting cuttings and maybe germinating seeds. A wooden frame lined with heavy plastic… or an old bathtub, maybe with insulation around it… or maybe there would be a way to connect a series of plastic buckets…

What do you think?

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Are you thinking about just using the heater or heating a jug. I feel like most heaters I have seen need to be submerged.

Yes, definitely using it submerged in water somehow or another, and then letting the water transfer the heat from there.

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I don’t know the relative safety of a space heater versus aquarium heater. I nearly burnt down my ghetto homemade greenhouse with my space heater this summer but it was the plug that failed, so you could easily have the same failure. I like the concept of having warm water under low hoops buffering the temperature swings. I wish I could afford a powerful enough pump to have the water for my pond pumped up behind the walls of the greenhouse to circulate in the warm air before it goes back to the waterfall.