Russian Vobla - ratio for the brine?
What you tried is nowhere enough salt for the fish. 

Both methods, drying it out in a bag of salt, and brining it in a saturated salt solution, are common. But this kind of high-salt brining is not similar to typical vegetable brining which is done with low-salt solutions. 

When they say a "saturated" solution, they mean it. The Internet says that this happens at 35 to 40 g salt per 100 ml of water. I have never measured it, but it is obviously a lot more than your 5 g of salt per 600 ml. 

There are also other ways of gauging the saltiness of your solution. I found a recipe with the directions to keep adding salt until a fresh egg or a cleaned potato floats on top. It also gave 48 hours as the brining time. This will probably have less salt than a saturated solution, but obviously, what you want is a very strong salt solution. I think that you can go with the actual saturated one (keep adding until some doesn't dissolve), it is unlikely to be an economic problem for you to add enough salt to saturate it.