you have to get the right type of lava rock ,, it needs to be 100% volcanic , and not that other crap ,, I have it in my canister instead of biomax and bioballs, and in my 2 hob marineland 50g rated filters aslong as your first media tray has a coarse sponge the second can be lava rock then third , seachem or activated carbon(start with small doses of carbon if just starting to use in DT) at 7.99 a bag 15lbs at Canadian tire its the cheapest alternative to live rock(Which is on average 6.99- 9.99 per lbs in Canada) and that's not even pukani or fiji lol its mostly base rock and diy quickcrete rocks that has been in a sump or developing tank for years