I have 2 of these and they are very mellow. I also have zebra dartfish and they all hangout together. They do hide a lot. They come out to eat and that's about it. But I like them anyway. You don't need to worry I don't think because your goby hangs out in the lower part of your tank right? These guys will hang out middle to top of your tank providing you have rock where they can hide. I didn't realize that firefish are actually dartfish too.