Water cows arent more aggressive feeders they are daytime fish MARBLE GOBYS are nocturnal fish with night vision that spend the day sleeping and hunt at night. Two different types of fish simular in looks but very different in habit. A MARBLE GOBY can eat everything a water cow can and then some. I had a 18 inch male that loved to eat baby red ear slider turtles, water cows attemt to eat alot of things but they also vomit alot of things back up(my dorado from jeff rapps and ashortnose gar) because the bite off more than they can chew alot of times. Once it goes in the MARBLE GOBY it doesnt come back out. I like both of them i have 4 MARBLE GOBYS and 3 WATER COWS. MARBLES get much larger and more aggressive most never recive a proper set up and care so never reach full potential. I hae mine cows in with 2 female MARBLES and they rule over the cows with a iron fist had 4 cows big female just killed ne 2 weeks ago. All my goby will feed in the day and hand feed also except the one in this growout will feed in the day but not from my hand yet. Most tanks i see with MARBLE GOBYS have them just sitting out in space these ae true nocturnal fish that thrive off of caves and hollw logs for daytime resting. They inhabit the same waters as giant snakeheads,giant knife fish and asian arows these gobys are one of the things that go bump in the night.