So for those of you wanting to use aquarium heaters to temper your water / chemistry, I have come across several different types that require different methods to adjust beyond their "maximum" temperature.
These heaters are all goverened by a thermostat that prevents them from heating water past a safe temperature for your fish. Which is always around 32C.
But the thermostat is capable of going above, or when using a PID controlled thermostat, disabled altogether.
In each case, the thermostat appears to be set with a screw on the end of the heater where the cord comes in. Turn one way and a plastic rod screws in to decrease the max temperature, screw the other way and the rod is pullup in the opposite direction.
They all use some sort of plastic limiter tab that prevents the knob from turning too much either way.
Sometimes they are built into the knob. So you remove the screw from inside the knob so it can be pulled off when it can't turn any more. And brought to the lowest temperature indicator, and you do that over and over again until you can't move it any more.
Others use a pointer on the knob that hits a plastic block. You can cut the block off or the pointer so there is nothing to stop it from turning all the way around.
Two other methods had internal blocks. One required pulling up on the knob once it reached the maximum and wiggle it to get past the block.
The other required the removal of the insides to cut out the plastic block. On that particular one, the insides are not epoxied or anything. Just set into a rubber bung to seal everything. Carefull pulling on the cord and knob slowly eased the bung out.
They're in a glass tube so don't put a knife or anything in it to pry it out or you'll just end up breaking the glass. Small $10 loss as that's all these things cost on eBay. But you can often be waiting for up to a month for them to come across on the slow boat.