puffa lover: I commend you for trying to fix the issue you are having. Sometimes, we search for the answers we want to hear instead of getting to the truth of the matter. There are always exceptions to every rule but you fish have different requirements--size vs. salt. One suggestion--instead of dropping all that money on a CRP or dragon eel, buy a bigger tank for your mbu and turn the current tank into a brackish water setup for your gsp.
Also, just wondering how the fire eel and mbu get along?...i was thinking about trying that combo.