Backyard Aquaponics
A place to discuss aquaponics

I went to Pebble Junction today to look at some different types of media.
Pebblejunction
What I expect that I will get is 1/2" brown river rock (it is the same stuff that I got by the bag for my barrel ponics system)
It costs $50.10 per ton (aprox 1.25 ton per cu yard) Delivery is $95 for up to 6 tons so the total would be $395.60 plus tax for 6 tons or 4.8 yards of the stuff.
The white river rock is a little pricier.

They have a couple other products that I thought interesting.

Crushed Coquina is only $48 a ton but the guy said it tends to pack down really bad so might not be appropriate.

And they have Washed Shell that is listed at 1/2" though the sample I saw looked bigger. It is $52 a ton and I was wondering if it would be a good media. I expect it is lighter than river rock and if it were to buffer to an appropriate pH then that would be great. However if it buffers more like lime stone then it wouldn't be so good. Anyway it would total $407 + tax for 6 tons delivered and probably come to more than 4.8 yards.

I spent much of the morning calling around to anywhere that seemed like they might have aggregate and some that carry exactly what I want but only deliver 22 tons or more others just don't deliver where I am. Finally the best price I found with delivery for 3/8" gravel was $329 + tax for 5 yards but the rock happens to be white lime rock.

Any comments on the different types would be welcome!

The limestone and the shell should both buffer to about 7.4 - 7.6 which is higher than I keep my system, but not bad, all told. They're both really calcium carbonate. Crushed coral would buffer for a while, but then go more inert because some of it is argonite, I think. In any of the above cases, you will slowly lose media mass.

Go with the brown river rock, and if you can get a couple of 40lb bags of the washed shell, add that to the growbeds to buffer as needed. I'd be afraid to use all shell in case I'm wrong about the pH range it will give you.

OK, found a reference that says calcium carbonate has a pH itself of 9.8. So I'd be worried that a whole growbed of it would be too much. The limited amount of shell that I have in the system still has to be helped out with some potassium bicarb. So, yes...go with the brown river rock, but keep some shell for adjusting pH.

I have found the smaller 5mm blue metal a lot easier to work with than the 20mm blue metal I had originally. (Say 1/4 inch v 3/4 inch.) It is easier on the hands and a lot easier to plant in. I haven't had any issues with it blocking holes in drainpipes either.

TT,
Yep, would love to get such. The 1/2" Brown river rock isn't too bad though, it's about half way between those two sizes.

I'm guessing that the closest thing we have to your 5mm "blue metal" might be gravel fines but they are like $97 a ton which is way more $$ than I want to spend on less than a yard of material.

Janet,
Thanks for the tip on the shells. I knew that Limestone would probably buffer too high for my purposes but wasn't sure on the shells since shell grit is something we use for the purpose of buffering without too much worry of going over. Anyway, I guess I'll stick with the river rock for the beds and next time I'm over near Pebble Junction, I'll take a 5 gallon bucket and get a few pounds of washed shell (heck it's only ten cents a pound.)

Janet, where did you find that info regarding PH?

i thought i had posted this elsewhere but i'll repeat in brief.

Have been doing some research on carbonates due to my imminent setup of a marine aquarium.

Calcium carbonate comes in three forms the two best known structures are calcite and aragonite.

Calcite forms the majority of sea shells, most terrestrial shells and limestone.

Aragonite is the form produced by corals, a couple of terrestrial shells and some mineral deposits.

Calcite will buffer to the mid to high sevens and aragonite will buffer to the low 8's.

Aragonite if preferred for marine aquariums as by the time (PH) calcite buffers the tanks PH has dropped far to low (marine ph usually about 8.4)

Regarding the minerals being rendered "inert" i have found data that says crushed oysters doing this because much of the carbonate is used up leaving the "binding" cant remember if it was chitin? giving the impression that the shell was still there................have not heard of this with crushed coral but may well be the case.

By definition limestone is still called limestone if it contains more than 50% CaCO3 but may contain 40+% dolomite (magnesium carbonate) which may buffer higher? i dont know.

I believe that marble is also composed of aragonite.

Hope this helps

Steve

That's what I thought I knew. Thanks Steve. I can't remember where I found the info on the higher pH. Maybe there's a difference between the pH of the substance and the pH it will buffer to.

I think it was Don the Fish Feeder who said that the structure of coral would be left behind.

So is the implication that washed shells would in fact be a good media? I do think you would slowly lose media mass, though. You'd have to keep some spare around.

If washed shell would in fact be a good media, please some one say so! I will hopefully be ordering my gravel soon and if the shell would be good, I'll order it instead. It should be lighter to handle as well as whiter (more reflective of the sun) and if it take care of my buffering then It would be worth $12 extra.

Or would I be better off just buying $12 worth of shell to use as buffer and putting the river gravel in the system?

I don't mind if the media in the grow beds looses a little volume over time but I would be pretty annoyed if the media disolved away in a couple years.

Keep in mind that I have no real good way of knowing exactly what shells are in the mix as it is generally sold as decorative mulch.

You could get a half-bucket of shells and soak them for a week. See what the pH in the bucket does.

Structurally, I see no harm in the shells. You will slowly lose media mass; I see it happening in the column of shells I have in the system now. I've lost maybe 1/4" height over a few months. If the whole growbed were shells, I think you would lose height more slowly, but I might guess that you'll lose a half inch a year of media. The shell pieces will also reduce in size, creating sand before it dissolves entirely. That could create some anoxic areas in the bottom of the growbeds, but that's not all bad either. See the RSG filter discussion.

If calcite (shells) will buffer to mid 7's as Steve says, that would be fine. That's actually the effect I was seeing from my crushed shells/coral before it wore out.

Given the lighter color and lighter weight, maybe go ahead and try the shells. As the system matures, the pH will try to come down and the shells will start to dissolve. Try it and let us know. You'll be the first!

Yep, I might just have to go back out to pebble junction and buy a few bucket fulls of shells as well as a few of the river rock. That way I can compare weight better and run side by side bucket pH tests. I should probably use water out of the system I have running as the pH is already down. My well water with no bacteria active won't help in such a test.

The bacteria is an un-necessary variable in the test to see what the media does to the pH. I'd still use the system water though since the pH is down.

Once the system is fully matured (6 months +), the organics will try to pull pH down and the shells would try to pull pH up. It remains to be seen what exact equilibrium it will hit, but you'll know that the maximum pH you could hit (without further additives) would be that of the bucket of shells.

Got some buckets of shells today and starting some tests!

Initial reactions and points of interest.

The shells are much lighter per given volume than the 1/2" brown river rock though they may be more prone to packing down over time.

The shells seem to need more washing (or another way to look at it is they cloud the water much more even after washing.) The river rock clouds the wash water a tan color while the shells turn water a grey white and even after some extra rinsing with the hose, still turn the system water cloudy.

I put a paint strainer bag of washed shells in the flush tank and will re-test system pH after it runs for a few hours.

I've also set up a few other tests.

System water test
water tanked after getting these readings 2/20/2008 1:50 pm
pH 7.5 (I added well water a few hours ago)
Ammonia 1
Nitrite 1
Nitrate 10-20
one gallon of system water for each sample.
2, 32 oz yogurt container fulls of media for each sample
Sample of river rock in one container with bubbler
Sample of shell in other container with bubbler

readings shortly after install
River Rock water @ 7.3
Shell water @ 8.0

Next test using collected rain water start 2/20/08 2:45 pm
start pH 6.3
similar volume of media in two yogurt containers and cover with about an inch of the collected rain water.
River rock and shell again
Readings just after setting up test
River Rock 6.6
Shells 8.0

it looks to me as though this shell is likely to buffer to 8.0 quite quickly. It will be interesting to see how that balances with the active system over the next week with heavy dosing.

My initial inclination is that the shell as the only medium in a system will tend to buffer too high for most plants to stay happy though it might be great for some types of fish. I will attempt to withhold judgment for a little bit and see what happens. The river rock may be cheaper but the shell is lighter. Hard to say which I want to win.

Readings this morning 9:40 am 2/21/2008 on the PH tests

System water with shells 7.6
System water with river rock 7.0

Rain water with river rock 6.6
rain water with shells 7.6

Just a note: what we get as "fines" out here is the stuff that ranges from about 1/4" (6mm) on down to dust and tends to settle into a rock-solid layer fairly fast once it gets wet. The smaller stuff fills the gaps in the larger making it very dense and impermeable. Much less permeable than sand. Maybe yours is better than ours..

I decided against fines as it is say too costly anyway. I'm more interested in spending less than $50 a ton.