The Puffer Forum

My GSP is about 1.5" long, and came from a fresh-water aquarium. Since I got him a week ago I've been gradually adding a little more marine salt to his tank. He started out in fully fresh, and I add about 1/2 tsp of salt per gallon when changing the water, which I've done just about daily - 20% or so. At what point should I make his environment fully brackish? And, will he acclimate to it all at once, or should I do it in steps?

Raising the salinity at 0.002 per week until the achieved gravity is reached is best. Look into purchasing a hydrometer, or a refractometer to help you keep track of the salinity. Hydrometers are cheaper but refractometers are more accurate as long as you remember to clean them after each use.

Thanks! I got a hydrometer and it is currently soaking in salty water as per the instructions. I'll be able to start taking readings tommorrow.

Looks like you are good to go.

The info you are looking for is in the GSP article in our Library.

A juvenile GSP will be fine at SG 1.005 (~9 grammes marine salt mix/litre) for at least a year. Once it's above, say, 8-10 cm in length, you can then gradually raise the salinity to SG 1.010 (15.5 g/litre). While there may be advantages* to maintaining these species in marine tanks, it is not essential.

Bizarrely enough, in the wild it is the juveniles found in brackish water; adults are reported by collectors to be entirely freshwater fish (see Fishbase and FAO documents, for example). The idea they "grow up in rivers and swim to the sea" is apparently erroneous. What brackish water probably does is reduce nitrate toxicity, and the additional carbonate hardness moderates pH changes. These are both factors that could perhaps be causing stress on fish maintained in purely freshwater conditions.

In any event, maintenance in clean, nitrate-free, stable-pH water is what matters. Focusing on the salinity is (in my opinion) not seeing the wood for the trees. It's a common occurrence in fishkeeping to connect a valid observation (e.g., Mollies do better in brackish rather than freshwater aquaria) with a bogus explanation (e.g., Mollies are really brackish water fish in the wild, which they're not). I'm as guilty of this as anyone, which is why it's important to keep going back to the primary scientific literature when trying to understand stuff rather than re-hashing hear-say picked up in old books and random forum posts.

Cheers, Neale

(* For example use of protein skimmers and live rock filtration/denitrification.)