Thanks for the responses
I will try the frozen Fish. Is it dangerous to feed frozen-thawed saltwater fish? Or should i better feed frozen-thawed freshwater fish?
And another question that i have is: My ph level is very alkaline, is this dangerous for the fish or can i leave it like that?
IME I don't think FW or SW matters. I feed mine whole SW baitfish that I catch myself in the ocean. Fish lose excess salt easy. Their osmoregulation is superb. If so worried, can soak the SW fish in water for some time before feeding.
The advantage of SW food fish is that almost all or all (?) SW parasites and other pathogens (bacteria and viruses) perish in FW and vice versa. That makes the food safer, on top of freezing killing parasites. Proper freezing (low enough temp and long enough time) is said to kill all parasites.
From what I read, gulpers need either acidic or neutral water, maybe they can adapt to slightly alkaline but very alkaline may not work out long term. You will see your gulper getting excess slime, losing appetite and wasting away.
Often people do report problems with gulpers and among those cases, most appear to be connected with hard and/or alkaline water. Again, such is my impression from reading around.
I have had three gulpers for about 1.5 years but it's my first hands-on experience with them. I always have been wary after reading people having problems with them and their water. Finally, I worked up the courage to give them a try. They are said to not be a beginner fish, FWIW.
What also still worries me is that I'm yet to read and see a mature adult or even old gulper that would have the supposed max size of 12"-14" or at least something close. Through the years I've read dozens and dozens of threads "Look ya'll, I've got a gulper or gulpers" and never a follow-through,
never a report of "Look, I've had my gulper 5 years, or 10 years, and look how big it is".
This makes me suppose that the vast majority of people fail to grow them into appreciable age and size.
Here is my recent thread on mine:
https://www.monsterfishkeepers.com/forums/threads/gulper-catfish-trio-7.678741/