Not been around here much of late but I have a question:

How low a tem can my goldspot pleco go to? He has been in the native tank at 20 deg all spring and summer and has grown well from about 6cm to maybe 15+cm (brought at about 4cm?). Now that the night temps are starting to fall the tank is getting down to about 19 deg by the morning. Should I move him back to the 180L guppy/planted tank (sitting at about 23)

When it stops moving it may be too cold for it

Sorry can't help with how cold they can tolerate but when my heater malfunctioned and found all the rest of my fish floating belly up in 41 degree water my goldspot was still doing ok.

They're pretty tough from my experiences - can you put a heater in there to keep it above say 20 degrees? Or are you trying to get a proper cold cycle for the natives?

I'm surprised it could handle 20 degrees and do well but in my opinion its not a particularly kind thing to do over winter. I would suggest moving it back to your warmer water tank, at least over winter.

I have kept them in aquariums without heaters indoors no noticeable issue, i dont believe however this is a good idea and not fair on the animal.

I would move him back to the guppy tank. Its a bit different when the days are warmer but once the days cool off, while it may survive it will no longer thrive.