Sorry it took so long to write back. We used to backwash both at the same time and then someone told us this was wrong and we backwashed one filter at a time. Tried to get into the sand filters and broke the handles of the tool.
Our pool is 34 years old - maybe the plastic tool got brittle. Got a new handle and broke it again. Last night we finally managed to open the filter tops and a lot of sand is missing. We added the sand and it starts to look better already. Thanks a lot for all your help. I hope it is looking clear tomorrow!