Worried about my fire belly toads

[Fbt123]

I have 2 fbts. They have recently been moved to a larger viv (45x45x45cm). However, they have gone from a light green to a very dark green colour. They have also been very lazy and sitting in the water with their heads poking out the water. I have gravel as a substrate and an aqua flow 50 filter. The filter may be scaring them as they have never seen one before. I am quite worried as they usually are a bit more activate (but not by much). I would be very grateful for any help, thanks. Also can anyone tell me about some good viv setups please as im thinking about changing it soon. Maybe putting in a piece of perspex in the middle then a layer of gravel and coco fibre soil on top for the land. Is this a good idea or would it stress them? Help!!!!