You must avoid the vinegar, that’s the problem. The coffee filter is one usual method. Another one that you should try is the following:
– Keep your vinegar eels in bottles with a long thin neck. Keep the vinegar level low so that at the neck’s end there is an air-filled room about at least four or five cm (two inches).
– Take a pad of wotton wool (normal medicinary cotton wool) and fix a thread on it of about ten cm (5 inches long). Failures can depend on false cotton wool or cotton wool pressed too heavily so that the eels can’t work their ways through it.
– Place the pad with help of a stick in that way into the neck of the bottle that it just touches the surface of the vinigar with the eels. Perhaps you must try several times.
– Now you fill cautiously water in the empty neck above the cotton wool. If you do it cautiously, only little mixture with the vinegar will happen.
– Wait for about half an hour. During this time the vinegar eels will work themselves through the cotton wool in the clear water above. You can follow it.
– If you think there are enough eels, you can pull the eel-filled water by drawing at the thread and pour it into a glass. Perhaps you must train this also.
– Then you have rather pure eel-water and you can give it intio your tank by using a pipet or small spoon.
This is a good method if trained a bit.