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Do you change the water even if it is looking great or do you leave it?
Depends. Brewman's point, I think is putting if off late, increases the chance you can make it to warmer weather before the next change.But getting back to your question, it depends. If the water still looks good after 4 months that is a tribute to your care. Try this, turn on all your jets and air flow. Let it run for 30 seconds. How long does it take to clear when you turn the jets off? Thats an indiciation of the amount of dissolved solids. Is that accompanies any unusual smells or gassing when the jets are running, I'd change the water now, if not, hold out as long as you can. I just changed mine and the water after fill up was 70 degrees so it didn't take long to heat up. In coming months your tap water will get colder and require a few more hours to heat back up.
Thank you allIt takes about 15 to 20 sec for the water to clear up after I run both pump's & all the jet's. There is no smell in the water I try to smell it all the time. There is no foam because I'm crazy when it comes to foam. I'm still using the same amount of chemicals. I wash my filter out ever week & scum busters out every week. I'll wait to after my son's birthday party in Nov because the little kid's will be in it & will mess it up I bet.