Red Cherries Strange Behaviour

[Alexarium]

Hello to all!
 
I have an AquaNano 40 tank and lots of red cherry shrimps.
Yesterday I noticed a very strange behaviour: many red cherries gathered at the out-take of the tank (this is where the water gets sucked into the filtration system.
Here are some photos to show what I mean:
 

 

 
 
It is an established tank. All water parameters are great, the tank is heavily planted. No recent changes at all.
 
Any ideas why this is happening?

 

[fluttermoth]

Two possibilities spring to mind...

Maybe some tasty morsel got sucked onto the sponge and they're eating it, or

Is that big one in the middle a female? She could be in breeding condition and the males are swarming round her. I know female shrimps release a hormone into the water when they're ready to breed and you'll often see all the male shrimps in a tank swimming about madly.

Have you tested the water, just to rule out any other issues?

 

[Meeresstille]

I always find shrimp in my filter, not sure but maybe they are after the organic matter caught in the filtration system.

 

[Alexarium]

Yes, the water parameters are great and it's more than 6 or 7 females pregnant, so I presume some other females might be in breeding condition.
 
The tank is like a jungle, it's really heavily planted. I actually had to cut some of the plants as they were creating so much shade that the tank went dark!
 
What I can't understand is why they would gather there when the tank is full of food (hence the population bloom).

 

[Liv15]

My shrimp just do that to clean out the filter

 

[This Old Spouse]

Mine do that quite often. Can't tell if it's feeding on a magical morsel caught in there or if they're breeding. Either way I don't think there should be any concern. Also, I generally find several when I clean out my filter. Babies get sucked up and then have a smorgasbord in the canister filter. When I pour out the old water, I always use a strainer of some type to catch them all, then just plop them back in the tank.