hi i think you are starting your sg a little high,i would bring it down some to start,or your going to eat up salt.poor mollies,i have a 40g brackish tank for just mollies sg at 1.020 and they love it fun fish to drink coffee too in am,there is alot of funky brackish fish but just choose whats right for what you have..figure8 puffer is cool for 20g but they are lower end brackish sg.i have 1 gsp in 40g at 1.020,i have a big vat of water changes made up in 50g rubbermaid for both my brackish tanks,works out better for me and the dwellers and i use marinland bio wheels on my tanks they always have did a good job for me.i found that you can help yourself with the filters by altering them for just the mechanical then get some little socks to put bio balls in,and carbon and seems to do pretty good for me.options are wide open thats whats nice about these forums you get to read more options then just taking your lfs at what they say.and i do like the tankoh yeah i would use a sand substrate i think its much easier mant.