Holding tub for White Cloud Mountain Minnow

I have just setup this holding tub in my greenhouse for my White Cloud Mountain Minnow that are still in the outside tub pond. It is a 35 litre "Really Useful" plastic box I purchased from Amazon for £10 delivered. These are excellent storage boxes and with having external support ribs I think they should be strong enough to hold water. I stood the box on a piece of plywood with bubble wrap on it ,to to provide a firm base. This should help prevent distortion. I have installed two airlift filters, the first is the Alfagrog filled one which has been running for a few months and the other is a twin sponge airlift filter. I have added rainwater to the tub as this is what the outside pond currently contains and the fish are used to it. The temperature of the water in the tub was 11.6 deg. The greenhouse air temperature is 19.3 degrees. I will transfer the fish into the tub once the filters have become established.