hot tub chemical help

First time hot tub owner and im not having fun... Its been running for a few months now with no issues other than fighting with chemical levels. I only use it 2 times a week and keep it at 80 when not in use. Its about 200 gallons so pretty small in comparison to others. Right now my chlorine levels disappear after 2 days of no use and i feel like that's not common. The chlorine I am using are large tabs that are good for 500 gallon spas (I break them into pieces first) but its also almost 3 years old, shouldn't matter though because I didn't have this issue when the water was new. My pH level is usually around 7.5 which is also a constant battle but usually only once a week. I check it with both strips and I bought a digital reader as well to make sure its dead on. The hardness is usually 300 or below, it drops a lot i think because im using so much chlorine but I feel its needed. and the alkaline is usually low as well around 80 or less, that is also a constant battle now that water is 3 months old. Google says the most likely cause of this issue is biofilm. My tub surface is not slimy and i really don't think the hose area should be either. The tub hasn't had a chance to sit without chlorine. IF this still could be the case, is there a way to test for biofilm without draining? I live in NE Ohio and draining and refilling is not a good option in the winter. Any thoughts?