I just read an older post here where the person had the same problem. She was told to add some weight inside the box. But I've been using these for about 15 years and they've never floating to the top.

Actually, I think I've figured it out. I think especially with the box in the 300 gallon pond, I may have not given the box time to totally fill with water, before I started it up. I decided to stop the pump and let water get into the entire box. I think if there's any air in it, the pump sucks in the water that's there, and doesn't let the part with air get filled with water.............so stopping the pump for a short while let it fill.

Thank you, catherinet, for all your help. lol!

Sorry, I missed the post! My container pond is shallow so I always top the filter box with a flat rock that does double duty to weigh it down and disguise it.

Oh that's okay PKponder. Some people have lives away from this forum. :) I was wondering........do you think the flat rock obstructs a fair amount of the water going through the filter? Maybe we have different box filters, but mine only draws water in through it's top.