That is with out a dout a vilot goby or dragon goby also know s the goby eel. they are normlly kept in brakish tanks, But over the years of being bred on fish farms have now been and or beening seen as more of a freashwter spiecies. I have one 3.5yrs old. there is a debate over " If keeping them in FW is hurting there live span or if brackish is the best for them." It's a he said she said type deal imo. But the darker gray coloring shows tht one was in fw and the other brackish. they can and do reach 12'' long. They have very large mouths and some say they will eat anything that fits into there mouth. But the one thing is there mouth is big but throat is to small and nearrow for them to et guppies and neons. Must fish are to big for the passage way. they are filter eaters. Scuoping up even gravel and filtering it with there gills. It's neat to whatch the gravel filter out the gills. They are very very blind do to there eye size. But can and do swim super fast either forward or backwards. Can stop on a dime and hit revers like that and be gone in a flash. best to throw in a peice of tubing for filter for them to hide. They mainly come out at night.