I catch both of those fish here where I live or a very close approximation of them. the flounders will live in freshwater for a couple of years but will eventually die, the gobies only come into freshwater to breed. they will not live long. .002 is as close to fresh as you need to be, very few aquariums are much fresher than that unless they are keeping something that takes very soft water. Even a teaspoon per gallon would be saltier than that or close to it. If you really want to keep these fish in freshwater add calcium chloride to the water, about a tablespoon to your three gallon tank should do it. I would add it slowly over several days a little at a time but make sure it is already dissolved. Calcium chloride will keep the osmotic pressure of the fish close to normal and the small amount of salt will satisfy any need for salt they may have. Most freshwater fish will do quite well with this mixture except maybe fish that require very soft water to breed. doing this the water will still be brackish but the calcium chloride doesn't hurt fish like sodium chloride can. I do this with many of the native fish i keep so i can mix fish that live in tidal creeks at low salinity with fish that like freshwater but live in the same area. I've even kept fish like cardinal tetras like this for many years. The key is allow the fish to have neutral osmotic pressure or as close to it as possible. just like distilled water will kill a fish too salty water will as well.
