It may not be the bio load exceeding the bacteria, but rather you may be killing bacteria whenever you shut your filters off for that time period. I dont know how long bacteria survives for when it doesnt got oxygen or water flowing through it. Just add more bio media, it may not be needed though because I am unsure if thats your problem, but it doesnt hurt to have it either.
If you do WC, just take the bio media and put it inside your tank to sit in there, or in a bucket of tank water.
If you do WC, just take the bio media and put it inside your tank to sit in there, or in a bucket of tank water.