They will slow down a bit in my experience, the stones an curtains get a bit blocked up eventually
perhaps if you changed the intake filter, for the pump?
I've heard this said before, but never experienced it yet. to me its hard to see an airstone, that's being used, getting blocked. though i have broken a couple.
i was wondering if they "crumbled" with use?
even my old air curtain, though very green(bought 5 years ago second hand), flows the same as the new one.
ok, i understand this does happen. just cant make out why i dont see it.
now the diaphragms and one way valves get gummed up and weaken, on the air pump itself. i find a good service (clean or replace one way valves change the intake filter) help. but diaphragms are not always available. so in the end you have to buy a new pump. the position i am in now.
I've found that "none return valves" dont last too long. they can cause real problems with the pumps flow. also causing back pressure, which stresses the pump. they go, in my experience, every 6-12 months.