I have used the colored round ones....
What I have are the square flat ones, not the round ones. I am concerned with whatever was used to get the rainbow colors. Is it 'painted' on, or actually internal to the substance itself?
Exactly how do you plan to implement them? Once I have your answer, I can better give you a solution.
My filter system consists of 3 whole house cansiter filters in cascade, for mechanical, chemical, and bio filtration. This also allows for palying around with the filtering method, such as coarse-fine-bio; or coarse-fine-chemical, etc.
I haven't decided exactly how I will use them. My ideas so far are to 1) shred them and stuff them into the filter, or 2) cut them into donut shapes and carefully layer them into the filters. In either case there will be a 1 inch pvc pipe, which fits nicely in the center of the filters, to draw the water out from the bottom of the canister. Might also layer them with gravel.
Sounds like overkill for a 55 gallon tank, but this is A LOT
cheaper than a store-bought filter rated for 55 gallons!
My new job with the post office has been sending me away for training for weeks at a time(in fact, I am at this time in beautiful Norman, OK), so I haven't had much chance to work on the system lately. When I get it done I plan on posting pictures here.