I have a cal spa the heater light is on but not heating up just staying at 78 degrees I have a meter but not sure which wires to check on my whisper power unit. There is also a long round fuse that is a bg3031 could that be the problem?

You have a meter, and you have a fuse... and you're wondering if the fuse might be the problem?

I've never figured out how to say this in a polite, friendly, respectful way (my apologies), but, if you haven't done a basic test to see if the fuse is good, or you don't know how to, you should NOT be messing around in there. Electricity and water kills. Electricity by itself kills.

Never the less, you probably have a bad heating element.

Ok for the person that responded to my ? The fuse is good and not sure where the heater element is, is it the round thing attached to the heater tube or is it he whole heater tube. All wires that are attached to the panel are good as far as amps

The round thing is probably a pressure switch. The element is inside the "hearer tube".