Another species I am look at in regards to Hermit crabs. By the sounds of it this one may be a little more fitting for an octo tank. But will have to do more research first.
Electric Blue Hermit Crab
Electric Blue Hermit Crab
You will probably find it months from now, dried in some out of the way place... I usually watch the level on the largest surface area, in my case, it was the main tank. Also, on the sump, it usually is a set level, so it might not show at all on the sump.
I never trust hydrometers, but then I am not one to rely on testing but I have had a lot of experience with many different tanks, so I trust my gut and water changes more than test kits... I wouldn't recommend my methods for someone just starting out - there is a lot to be learned from watching a tank cycle.