I'd bet almost anything you haven't seen a violet goby live for ten+ years in freshwater. It just doesn't seem to happen, and that is their real lifespan.
I believe that every story of someone keeping a violet goby in freshwater is a fluke. They can go for years in fresh and appear totally healthy. Of course sometimes that isn't the case. Sometimes they have problems right away. Namely, people have described their gobies looking like they have excess slime when kept in FW. It goes away with salt. Someone else in this very forum had that problem recently. I checked in to see if their problems had persisted. When the goby was introduced to brackish water, it became much healthier. After time, it has been said that those kept in FW are susceptible to red tumors or ulcers that spread over the body until killing it in some way or another.
I could probably go on and on about this, but I'm going to stop and tell you why else your infatuation should take a back seat to your better judgement with this fish.
Aside from the issue of salt, the fish gets over 24" long as an adult. I wouldn't recommend keeping something like that in a tank less than 100 gallons. That may sound excessive but they can't have too much space. Their soft, scaleless bodies and delicate fins make it important that they be kept on a sof substrate like sand as opposed to normal gravel.
Although peaceful at first, mature violet gobies are territorial. Unless you have a huge tank and multiple hiding places, there will be violent battles that can result in death, if you're keeping more than one of them.
You'd have been better off posting in the brackish forum. That's where people with experience would be. If you're still considering buying one of these, feel free to ask any questions you have.
