Sometimes there is a need to clean your tube, you may have got algae growth or mineral deposits inside the tube.
Empty your tank, fill it with tap water and empty it.
Buy a couple of litres vinegar and do a 50-50 mix with cold tap water and start the pump and let it run trough your system over night, check the status next day and run it more if needed.
Repeat if needed, and go with a 75-25 mix if the first didnt do any change.
You do not need much, it´s better to have less collant when cleaning, more effective. Loop+tube is about 1.5 litres. Try with 2L to keep the pump submerged.
Do not use your laser with this mix in the tank, not likely bad but to be on the safe side.
Be careful with vinegar or other additives or stronger fluids to clean your tube if you run it for longer periods. It may affect plastic, rubber and metal parts in your water loop and pump.
Thanks to Akos Balog for mentioning this!