"I don't have alot of $$ so this is better then having those expensive filters that are already put together. I have a system that has 3 filterings items, 1 is the wheels, 2 is the filter you are suppose to put in it and 3 is the one you can put in to your choice that goes in the back. Anyways, when I first put this product in my tank in these areas these filter pads were to thick and it caused Ph levels to go way low and didn't filter right. So I then like a previous review 1.)I cut the blue filter from the old trays, 2.)I cut what I needed from the Mairneland filter pads, you will only need the one piece for both filter replacements 3.) I peeled the two pieces apart, to save even more $$ 4.) put in my carbon and zeolite crystals- what the directions called for, in the tray of the old filters 5.) then I sewed the new filters to the trays (don't worry the filter pads come off when you pull on them- I don't sew them too tightly). 6.) then I put the new and cleaned filters back into the filter system. And so far after a few days this method is working better."