A simple screen will prevent this. Make it larger than you think so is does not get blocked by algae or other stuff.
I once had a large turbo snail get in a drain pretty much blocking it. It worked its way down to the valve where it could not get out. I had to stuff a flexable rod down and...
Are you sure those are gorilla crabs? The one I had was much fuzzier and white. Also it had one large claw and one small if i remember correctly. Even the little ones have no real fuzz compared to the one I had. It was definatly preditory. It came in as a very tinny hitch hiker on some live...
I have 4 Tunze running currently and one gyre. Tunze's are 99% of what I have used for 25 plus years. The gyre is my newest addtion and i have to say it requires a lot more maintaince than my Tunze. The flow it produces it obviously different.
I also have some neon green toadstools and green frilly mushrooms available. The leather in the photo with the encrusting gorgonia is about 6", although not fully open in the photo. The mushrooms are 3-4".
If your using 2 part, i have calcium chloride in 50 pound 7 gallon buckets for $45. I can do smaller qualities as well. Also have Kalk at $3. a pound, 5 pound minimum.