indrpd

All my tank koi fish constantly swim against the filter water return.* As if they like to receive water current on the face.* This behavior is new.* Kois are bottom feeders.* When everything is well they lie flat at the bottom of the tank.* But since 2 weeks they all line up right below the filter water return sprinkler.* *They only stop this behavior temporarily when they see humain beings &* when they eat.

Can this be a symptoms of some uncommon disease?* I have read on the internet different aquarium fish symptoms but I have never heard of anything on consistent swimming against water current.

I have a theory for now: The fish have parasites attached to their gills & they try to get rid of them by receiving strong water stream thru their mouth.* What can you guys think of?

I also think maybe put a strong powerhead in the tank to see if they will spend their whole day staying in front of it.

By the way has anyone been to a vet clinic to treat their fish?* I am thinking of doing it.

About the tank:
established filter, ammonia nitrite=0, pH consistently at 6.6-7.0,aeration is OK, 45USGallons stone indoor pond, no lighting system, no plants, 6 baby kois & two 9 inch adult kois.