OK, first, promise me you weren't dumping the salt from the box straight into the aquarium. This will at least stress the fish, and potentially kill it.
Next thing, moray eels need fairly high salinities. Since you cannot measure salinity directly, you have to go by the specific gravity (density) of the water. For this, you should use a 'hydrometer', which you can buy from any tropical fish store that carries marine fish. The specific gravity you are aiming for is at least 1.010, and ideally 1.012 to 1.018.
The box of sea salt should say on the box how much salt needs to be added to a litre of water to make "normal" sea water. This is typically around 35 grammes of salt per litre. So if you have a bucket holding 10 litres of water, you need to add 350 grammes of marine mix.
Slightly complicating this is the fact that once the box is opened, the sea salt absorbs moisture from the air. So if you simply weigh out the salt, you are likely to be adding a bit less salt than you imagine, i.e., 35 grammes taken from the box will actually be 34.5 grammes of salt and 0.5 grammes of water. So any sea water you make up will be slightly under-strength. For brackish water fish, this error is unimportant, but still, try and keep the opened box in an airtight container (e.g., a Tupperware container).
Now you know how to make up normal seawater. To make brackish water 2/3 the strength of normal seawater, add two buckets of seawater to one of freshwater. This would be ideal for a moray eel. Similarly, one bucket of seawater to one bucket of fresh would result in half-strength seawater, in my opinion at the low end of what is safe for moray eels.
Use a hydrometer to keep track of what the actual specific gravity is; with each water change the specific gravity will change. Most brackish water fish tolerate changes in specific gravity very well, and in fact in some cases these changes are therapeutic. But you do want to give a fish time to adapt. Euryhaline brackish water fish can, generally, be taken from fresh to fully marine conditions within an hour or two, after all, that is what they experience in the wild. But the filter bacteria will not tolerate such a change, and you will probably need to adapt the salinity of the tank slowly to allow the "freshwater" bacteria to die off an be replaced by "marine" ones, and that could take several weeks. Obviously, using a ntirite test kit is a good way to keep track of the maturity of the filter as the days pass.
Cheers,
Neale
PS. The
Brackish FAQ has some useful info on these fish. One warning in particular that was passed on to me concerns the sensitivity of these fish to copper medications.
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are you using a hydrometer to measure the SG (ie amount of salt dissolved in it) of the water? I havent kept morays so cant tell you what SG you should be aiming for but you should really find this out.
