oyster shell stupid question

Okay, as I mentioned in the title, this may be a really stupid question, but I thought I would ask anyway.

Bio-balls are used for filtration and some say to get rid of those and go with more live rock. So, I was looking at making some DIY live rock and the website Garf.org says to use crushed oyster shells for addition to the concrete.

If the oyster shells are used for making live rock, why cant bio-balls be replaced with the same shells? Crushed oyster shells are cheap, and it seems if you can have those in the wet-dry filter, it would be the same effect as having the liverock.

Just curioius because the shells are available everywhere and are really cheap. Maybe you get what you pay for, but I had to ask.

 

I think I understand what you're saying.

The thing with bio balls is they are very efficient. They are an excellent filtration medium providing tons of surface area for bacteria to colonize. The problem is, they stop after nitrates are produced in the nitrogen cycle. There's just no way to create zones of little or no oxygen where the proper bacteria that conver nitrates to nitrogen gas can grow.

Using the crushed oyster shells in the DIY liverock recipe makes a porous rock rather than a slab of portland cement. With the oyster shells being placed in with little rhyme or reason, it makes it more possible to have low/no oxygen areas inside the rock much like liverock does. Liverock works as filtration because it's porous. It also works well because the bacteria can better convert nitrates to nitrogen gas when those nitrates are produced close to the oxygen deficient areas within the rock. A wet dry filter filled with bio balls would probably make way more nitrates than the rock in the tank could process.

I wouldn't put crushed oyster shells in my wet dry filter as I think they would trap way too much detritius.

Hopefully I understood what you said and my explanation applies.

 

That's what I was thinking. EDIT - that is the main reason people don't like to use it as substrate as much anymore.