I just got my 55 gallon fw tank established and now I'm hooked, next I want to get a 20 gallon long and get a figure 8 puffer and some gobys and maybe a couple mollys, my initial plan was to get a small sponge filter and put it in my 55 to get some bacteria on it to instantly cycle my 20 and then raise my sg a little every week, my main concern is mixing the brackish water, how difficult is it to get it right? After I have my tank where I want it do I have to check sg at every water change or could I just throw a cup of instant ocean in my new water and add with no real concern? Also what substrate? My tap has neutral ph so is that okay?