If you dose h2o2 directly to a system you will see an immediate, initial, drop in orp. It will then rise to slightly above the level it was at prior to dosing and start to even back out. The whole process takes about 4 to 6 hours. At 1.5ml per 10 gallons of 3% h2o2 you'll see a 40~50 drop. Using higher strength would only result in a bigger decrease.160 gallons.
Dave
Using an Oxydator and seeing a big drop would mean you've dosed more than the unit should be able to release at one time. While I don't have one yet I've been thinking about it. So my thoughts here are just an assumption.
Was the unit securely sealed when added to the tank? Did you verify the dilution of the h2o2 was correct? Also from what I understand is a W unit is a bit big for 160 gallons. An A or 2 A's would probably work better. One thing you might want to try is using 3 to 4% h2o2 in there and give it a go. The catalyst will react with less intensity so less peroxide will be added to the system, especially right after it's filled.