Help Me With My Watermelon Crabs

[jash93]

Hi I'm New To This Forum And New To Taking Care Of Pet Crabs...
I Recently Bought 4 Watermelon Crabs Which They Are All Males~
I Can't Find Any Information On These Fiddler Crabs...
So I Had To Ask Here...
I Now Have A Tank That Is 18" x 8.5" x 8"...
I Put In Some Sand And Some Fresh Water In It...
I Made It Like A Slope So That They Could Access Land And Water...
I Had Some Plants And Rock For Decoration...
I Was Wondering Whether They Need Brackish Or Fresh Water...?
And I Don't Know What Food Do They Eat...
Lastly I Want To Know If These Watermelon Crabs Are Native Hong Kong Because I Live In Hong Kong And Do They Have Any Tank Mates Besides Other Fiddler Crabs...?
Anything Else I Should Know?
Thanks In Advance And Sorry For Asking Too Much,
Jash