I've seen two common reasons for foam.
1. from suits that were washed in the regular laundy. The best laundry soap leave residue behind. Run your suits through a full cycle with no detergent and ask that any guests do the same. Or just go without suits.
2. Filter cleaner products can leave behind residue that you really need to rinse out thoroughly. Have you used a commercial product on your filters. When I use something I rinsh the beejeebers out of them till the water runs clean while I'm rinsing them and then I sit them aside and rinse them again later till they show no side of soap.
Chas mentioned change the water.
No, you aren't expected to do that every 3 weeks, but we have been discussing a philosophy (soon to be called Chas' law) that all new owners should change the water after a month, then hopefully every 2-3 months after that.
PS - No foam is the normal amount.Search the archives for discussions about foam.
Chas mentioned change the water.
but we have been discussing a philosophy (soon to be called Chas' law)
It isn't anything like Murphy's law, either!!!
Change the Water.
Change the Water.
Change the Water.
It was just a passing thought....
Also, Jason is correct: water which is soft has much greater tendency to foam.