I have three Yoyo Loaches in a twenty gallon tank with 2 dojo's, 2 gouramis & 1 small chinese algea eater. The tank is about seven months old and I started out with just two yoyo's. Since I've had them one of the yoyo's had a strange habit of not always swimming regulary but twitching, almost like a convulsion wherever he swam. Now all three of them do this occasionally or on and off. I do weekly water changes and other than this they seems healthy and happy, eating and playing although their sleeping on their sides gives me an occasional fright. Is this twitching behavior normal? and if not what could it be a sign of?