i'm currently converting my FW tankto brackish,for i have bought 2 GreenSpotted Puffers.
I been trying to get some correct info on a dutch puffer site, but the thread is becoming the longest in the whole forum and i got none the wiser.
I got some excellent info from a guy called Win, a former member of OFL,on converting tanks and i'll find and copy it for you.
I'll PM the post to you, if you are thinking of converting the filters, instead of going SW all at once. Let me know please...
All i can tell you there's a big difference in converting a tank to brackish, which will have a Sg (salinitylevel) of 1.010 -1.021, or any higher Sg, which will be ture salt water.
Fresh water is 1.000Sg
You FW filters will need to be converted very slowly, with one senseo cup of marinesalt at a time, every 2 weeks, for the change of bacteria could lead to a massive nitrate spike.
Sg should not be raised with more than 1.002 every 2 weeks.
And to be honest, my fish should be dead allready, for nitrites peaked into lethal levels, but fish still are doing very fine and growing like crazy...
The whole cycling proces will take at least 8 weeks.
But the fish you mentioned are full salt water, so in my opinion it's best to get live rock and a proteinskimmer, cycle the tank beforehand, and when levels are correct, put the fish in.
Just like with new oscar tanks, only do you test Sg, next to Ph, Ammonia,Nitrites and nitrates.
Remember keeping SW fish is an expensive hobby, if you want corals and anemones in there, for you need massive lights for those.
And do not underetimate the amount of salt you will need.
SW consists of at least 30% salt.That's a kilo of salt to every 3 liters of water!!!
But using a proteinskimmer and liverock will make sure you only have to do 5-10% wc every week, in contrary to brackish, which will need the usual 50% wc a week.
And you need a Sg tester for sure.
Please state any other questions, and i'll happily answer them for you.