safe for freshwater zig zag spiny eels?
It’s safe to use but the water is the issue.
Hello; Are you using or about to use this medication? I may be wrong but have been assuming you do not yet know what is causing the death of the fish. Throwing in a medication at an unknown problem can add additional issues without doing anything useful.
The general consensus about the water test strips is they are not very accurate. Many prefer the liquid test kits. If you do get a better test kit may I suggest the testing of both the tank water and the source water.
Based on your posts so far only, I am more in agreement with water quality issues being the main issue. At the very least steps such as more frequent WC generally do little harm. The exception I can think of being the case of a big difference in the pH between source water and tank water. With the feeding schedule and two weeks between the WC you report this may possible. Also the test strip, if accurate, is indicating a low pH.
Even in such a case the WC can be helpful if done in stages. Say 20% WC each day for a couple of days, then 30% for a couple of days and so on until you get to 50% or so. Then the WC can be at greater amounts if done often enough (say at least once a week).
I also find twice a day feeding suspect. This can and has led to a lot of excess uneaten food. May I suggest an immediate two or three day fast with at least two fasting days each week after that. Also only one feeding a day.