I've been using about 8-9" of pea gravel over 3-4" of large lava rock for the last year and a half. I think the pea gravel doesn't allow for enough aeration and am thinking I will change it out for 1 - 1.5" lava rock as best I can. I have some plants in though I have been holding off on putting in new. My system is live. This will be imperfect at best. Has anyone done this before. Any pitfalls I might not know other than I will be building a new bacteria base. I'm hoping the lower lava rock will carry me through.
Please share thoughts and experience.
I have a similar situation. I never completely filled my GB when I set it up. I have 6-7" grow stones in my GB now and am going to add another 3-4" of expanded clay. Moving the plants is an issue but they never produced anything anyway. I've just been using them to clean the water but now I want to grow some veggies. Bacteria loss shouldn't be a big deal for you since you are retaining some of your media and you have a cycled system. New bacteria should grow fast. I've found that I can screw up a lot and my system is very forgiving.
I have changed out several grow bed in my system over the years and have had not problems. I have many gravel beds with egg gravel and pea gravel on top. My gravel beds with pea gravel are getting gummed up with fish waste, so I agree that the small gravel does not allow for air since it gums up easily.
I have also used lava rock in my beds but recently heard that lava rock can create dead zones because of the many pits, and the wrong bacteria grow. I just seeded worms hoping they will eat the waste.