You might be between two stools as 300-400l is more than domestic / laboratory sized, but pretty small for commercial applications.
Is that the acceptable range of temperature variation or the range of set points?
How accurate does it need to hold the temperature?
How much energy transfer does this mysterious body of water need to keep cool or heat up? Makes a huge difference.
What fuel is available?
Is there mixing going on in the body of water?
Is there flow in and out or static water volume?
what happens to change the temperature up or down?
Is there cold or hot water available to use already?
Anything you do find will need all this information so better to start thinking about it now.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
Measured where? At one point, selected for a reason? At any arbitrary point? "Everywhere". Is this an average, or a uniformity requirement?
If you are relying on natural convection currents for mixing, then the "body" shape and design of the heating and cooling elements becomes super important.
You've almost provided enough information to start calculating the power and energy requirements. With tank (if it is a tank) geometry and insulation you could have a ballpark calculation for both.
So if your acceptable range is 10 to 30 C and ambient temp is 10 to 25 C why do you need anything? The assumption is that there is something which either generated heat or uses heat in this water bath but you're not telling us what it is or how much energy is involved.
You haven't answered any of the questions which we can use to point to to a potential solution...
Or just pump the water around through a radiator and fan unit?
Oh and one C accuracy is quite difficult unless you are actively circulating the water quite a lot. That sort of water volume is quite small so could easily vary by more than one C when heated or cooled.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.