Firstly, insulate your nute tank. This will make the amount of energy you need to add or remove less. This makes your job easier.
At the core, this is a problem of moving energy from where it is (your nutes) to where you want it (not your nutes). This is either done passively or actively. Passive would be to have something cold around your nutes so the heat just flows out of them into the surrounding mass. Active would be more akin to using a chiller directly on your nutes.
The ultimate budget cooler I can think of would be to blow air over the top of your nutes. The evaporation will reduce the temperature through the swamp cooler effect. You can get aquarium clip on blocks of fans designed to cool aquariums for about £10.
More effective but more work and cost (and less discrete) would be to get some old hose, curl it round in your bath, then fill the bath with water that is either cold or hot depending on whether you need to heat or chill your water. Run your nutes through the hose, it will heat or chill your water. If you keep the tap on and let the bathwater overflow out to the drain it will take the heat with it and be replaced with cooler water.
More effective but more costly would be to buy a tank for the purpose (which could be insulated) and use stainless steel piping (which will increase heat flow from the nutes). You could fix it to your mains pipes and have it run to your bath overflow.
More effective but more expensive would be to make some device to actively remove heat from your heat exchanger so you don't have to have the hot or cold tap running. If you go down this route google 'duty cycle' when looking at your active heat removing device.
At this stage, you are not far from the cost and usability of just buying a nute chiller but whatever you can build will not be nearly as reliable or effective.
And so the next thing which works better and is more expensive is buying a chiller. I just looked and they can be got for about £70 so you should only be spending a portion of that so it is worth having something less effective and efficient.
You have not said, what is your budget?