I first like to apologize for my first post for it doesn’t answer much. I don’t know why but I just wasn’t getting what you wrote and that would be partially to the watchman/sleeper being tied together.
I would first like to note that now all watchman or sleeper gobies are sand sifting gobies. Actually most of those gobies just burrow in the sand but eat food suspended in the water column.
When it comes to sand sifting gobies I would not recommended them for a smaller tank. The reason is that it is very difficult to get these gobies to eat prepared food and seeing that this 3-5 inch fish only eats whatever it can find in the sand should tell you that it requires an ample amount of sand to sift through. Also this sand needs to be very alive for it to actually get the food for it to survive. Unfortunately in a newer tank and especially a smaller tank the food runs out quick and the goby will starve to death.
Some however have had success keeping them and getting them to adapt to frozen suspended food. However if one does this what was the point in even getting a sand sifting goby? If you really are keen on having a sand sifting goby check this out.
The care and feeding of sand sifting gobies.
I hope this helps you out a bit.