Perp

We purchased a 700L two-person hot tub 5 months ago. The regimen recommended by the seller is a mineral stick, 10g granulated chlorine once a week (dissolved in 10L water, pour off chlorine, don't put granules in), 10g spa shock after every use (and at least weekly if not used more often), absolutely no bromine. Adjust pH and alkalinity weekly per test strips, test monthly at pool shop and adjust pH, alkalinity, and hardness. The first 3 months we had no issues - water was always pleasant, only required minor adjustments. Then we went away for a week. Having chlorinated the day we left, and leaving it to filter daily, we anticipated no problems, but when we got back it smelt absolutely rank - like a fish tank. (Algae?) Took a sample to the pool shop and they said the water was fine - pH, alkalinity, and hardness all in acceptable range - despite the stench. We tried chlorinating it a bit more - added an extra 20g - and the stench didn't go away. So we ran the recommended pipe cleaner through the pipes, emptied and refilled the spa, made sure both filters thoroughly clean, put in a new mineral stick. New water got smelly within days, and the pH shot up sky high - over 9.0! I got a terrible rash all over, my doctor thinks from the high pH. Spent about a week adding acid every day, did pH test, was down to around 7, then the next day it would be back up to around 9. 😒 Pipe cleaner, empty, refill again, and the same thing is happening - it smells fishy and the pH shoots up to 9 every day. The shop that sold us the spa tell us to just keep testing the water and adjusting it, but it seems to me that something must be broken or not working for this dramatic change in behaviour, and for it to continue going drastically wrong every day. What might be broken to cause the pH to keep shooting up to 9?