people have had success using carbon in a filter sock and placing the sock in a high flow area of the sump.
This is not ideal however as the carbon will eventually grind itself into dust.
Rox 0.8 is a branded product. It is a peletized product that offers extremely high surface area, while minimizing dust and grinding.
No matter where you buy it from, rox 0.8 is rox 0.8. Companies buy it in bulk, and simply slap their logo on it. So find the cheapest source.
Honestly, i would just buy a reactor. You can get one for around $50 and you will thank yourself immediately.
There are no fast fixes in this hobby, and if you are going to do something, you may as well do it right.
Also as someone pointed out, the attribution that carbon leads to any ailment in fish is merely anecdotal, and has zero scientific evidence supporting it. Millions of reefers use carbon because of how successful it is at what it does. Any one who does have adverse effects in livestock could probably trace the issue back to a number of other sources.
Go with the reactor, use rox 0.8, and enjoy the results.