@somethingfishy76 and I just got onto it. We put 5 litres each in our tanks. I have some in the inflow chamber and some in just before the return.
I've kept them in their bags so that it makes it easy to give them a jiggle every now and then.
Just have to wait and see how it works.
The jiggling is very important, I jiggle mine with a 25mm PVC pipe off cut.
When you jiggle the Siporax a few million bacterias will be released into the water column feeding your tank.
Furthermore jiggling Siporax will speed up denitrification. The more you jiggle, the less nitrate and phosphate in your tank.
As far as deployment goes, some prefer a reactor chamber , some prefer a basket, some prefer an egg crate basket with Siporax neatly stacked.
In my case 10 liters sips in my ~300 liter tank, deployment happened in the coast to coast wier. The mesh bags were discarded.
As far a sizes go, I think a 15 fits inside a 25 / I very much doubt there is a difference in performance. I think it comes down to preference and space in your sump etc - there is also a mini version to try.