I have the Fluval Evo 13.5 also and have the Sicce 1.0 pump in it. I've had this tank for about 6 months now and have it pretty heavily stocked with corals and CUC, but I only have a clown, firefish, and Royal Gramma. The clown has been in there for several months, but just added the other 2 a couple of months ago.
All 3 fish have been basically hiding all the time except when feeding. The firefish literally hides deep in the rocks and I only can tell she is alive because I can see way down in a hole with a flashlight. I've been through hundreds of scenarios trying to figure out why they are just totally hidden, and after a bunch of various testing, I'm convinced it is the strength of the pump. I re-installed the original fluval pump for a few weeks and the fish came out and showed themselves nicely, but the pump was not strong enough and I got all sorts of crazy algae in the chambers. Even with the low power of the original pump, the y splitter and the flare nozzles of the return pumps caused so much flow jets I think the fish hated it.
So, now I have put the sicce pump back in. I am having great luck with the sicce input dial turned down about half way, and I have one of those 3D printed random flow generator nozzles pointing directly at the back glass and a huge rock. I have a Aquamai KPS wavemaker on the left back side, so that basically the wavemaker is pointing at the flow from the return and 'cancelling it out'. Now there is almost no flow at all in the front of my tank (torches just barely move) and my fish are loving the front of the tank. They are out all the time now, and swimming all over. They don't go anywhere near the back of the tank because it is like a tornado back there.
Basically my take away: The Sicce 1.0 pump is really nice (quiet, reliable, etc), but it is WAY too powerful for this 13.5 gallon tank. Now that I know the flow is the culprit I'll be searching out smaller pumps in the future for this tank.