I wouldn't worry too much-these toads are curious and rather comic in some of their behaviours. As long as they can't be trapped under there or sucked into the filter in any way, they might just like the feel of the water moving around them. I have a small filter and my youngest toad likes to sit on top of it...I honestly think he likes the slight vibrations. He'll even kick the bigger toads away just to keep it to himself!! I am sure your 2 will soon enjoy the whole tank-once the novelty of their own filter wears off!!