It normal for.the heat to go back to the tank, the copper block is suppose to absorb the heat and take it hot water to the radiator with the fan to cool the heat, since the tube came off, the heat goes back to the block which is suppose to carry the cool water to the tank.
Usually temperatures in cellars are pretty constant (if it is underground)?
I can confirm that even branded heaters can cook the shrimp (learnt that the hard way), it happened to me so a controller is a good idea. If you are that bothered about the temperature being the same and the cellar does fluctuate a lot you may also need a cooler, depending on where you are based?
Very sorry to hear about the tank overheating and wiping out your shrimps! Can you get a 2 plug adapter and have the heater and cooler plugged into that overheating safety device as extra security? I now what happened is probably just a freaky thing (who would expect a cooler to heat the water?) that probably won't happen again but you won't be happy with loosing the crabs????
Hopefully the shrimps will be ok with the micro crabs (beautiful red wine in the picture)?
Most of the stringy algae has gone in my shrimp tank so I will do maintenance tomorrow, tests at the weekend and if all is ok I will order the (final) red shrimps next week.
Thumbs up for perseverance!