How big to Amphipods get?

[WallysWorld]

I have a bunch of fast movers on my rocks. Too fast to photograph so far. They look like amphipods, but a couple of them are 5-8 mm long.

How big to Amphipods get, and could possibly be something different?

I'll try to get a photo of these fast moving critters.

 

[Epic Aquaculture]

I have a bunch of fast movers on my rocks. Too fast to photograph so far. They look like amphipods, but a couple of them are 5-8 mm long.

How big to Amphipods get, and could possibly be something different?

I'll try to get a photo of these fast moving critters.

 

[WallysWorld]

That's creepy! My tank is only 35 gals, hopefully whatever these things are they don't get that big!