Clowns can not eat for up to 2 weeks.
Most of the time they are ok but to be on the safe side I'd get some garlic extreme and soak the food in it. On the water canges I'd go to doing 20 to 25% a week and see if it will drop.
Emporer Bio-wheel (not sure what model but big enough)
Actually here is the problem, Get the bio wheels off the filter. In a Sw
setting they become a nitrate factory. Take them off for a week and then test your water, I'll bet the nitrates will drop.