How can I heat up the water in a fish tank to a certain temperature?

I want to heat it up inside the water, not like a pad. I also want it to be flexible like up to 75 degrees and as low as 50 and I also don't want to spend more than 50 dollars. I need it for a few tanks, not just one so it has to be a pretty custom idea. I'm doing this for a college experiment for marine... show more I want to heat it up inside the water, not like a pad. I also want it to be flexible like up to 75 degrees and as low as 50 and I also don't want to spend more than 50 dollars. I need it for a few tanks, not just one so it has to be a pretty custom idea. I'm doing this for a college experiment for marine biology.
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