From here on you simply fill up your barrel full of good fresh reverse osmosis water and add your buffer to it, I think its also a good idea for you to have air running through the water via an airstone or have a circulation pump in your barrel.
TDS meter I recommend HM Digital
Buffer for Caridina SaltyShrimp Bee Shrimp Mineral GH+
Buffer For Neocaridina SaltyShrimp Shrimp Mineral GH/KH+
Cheap RO Unit Like Mine Reverse Osmosis Water Filter
And more cheap Revers Osmosis Units That won’t break the bank.