I have not used oxyclean in a holding tank
but we do use it in brewing. I suggest combining it half and half with TSP. It will work much better. It works very well at getting calcified deposits from the brew water
off I assume it would do the same thing for holding systems. Its also great for removing smells so I assume it would work pretty well for holding tanks
Normally, it is followed up by a good acid wash but the stuff we use in brewing is no good for delicate gaskets. We have special ones used in industry that easily resist harsh chemicals, don't pick up smells, or absorb colors.
BTW - don't close up the tank - the oxyclean will build pressure.