If it is not in contact with the water, it shouldn't be a problem (thinking about the regulator end here). In water, copper and zinc will slowly dissolve. This dissolution is accelerated the more acidic the water. Since CO2 increases the acidity it would increase the dissolution.

In all, if you feel you have to use a brass fitting I wouldn't worry about the levels of copper and zinc - they should not become a problem. However, I'd suggest an alternative if using it will cause you to worry in the future.

Here is an old article discussing the dissolution into standing water. Their concern is exceeding drinking water standards which are quite low: Trace Metal Contamination From Brass Fittings on JSTOR

 

Thanks for the reply. I attached a couple pics (not my pics) to give a visual idea of where the brass fitting would be on either a Rex Griggs or Cerges CO2 reactor.

The first pic, the water flows from the top nylon barb fitting, down the PVC pipe and out of the bottom nylon barb fitting. (From top to bottom)
The second pic, water flows from the right-hand side barb fitting (B), through the Tee (E), through the rest of the fitting/valves, down and up through the reactor housing and exiting toward the left-hand side barb fitting. (From right to left)

The only brass fittings you see would be in identical locations where I would have placed them. So from the locations of the co2 inlet and based on the water flow, I don't think there would be a constant stream of water flowing against the brass fittings, but I would think at the very least, there would be constant moisture on the fittings (probably some splashing too), so maybe not the same rate of dissolution as a higher flow constantly in contact, but still some.

Is there any known amount of how much copper would disassociate over X amount of time and what copper levels may be harmful to fish or plants.
I did hope to be able to house inverts (snails at least, but possibly shrimps as well).


Only non-brass fitting I could find locally that was a suitable size for the CO2 line (standard air line tubing size, 5mm, 1/4 OD, 3/16 ID?) was this quick connect fitting typically used for R/O water systems. It fits, but I'm not all too confident that it creates an air tight seal (leak co2, or maybe even water if pressure in the reactor builds up too much, maybe even popping off entirely?). Anyone know if it's a good option? I have seen some CO2 regulators (CO2Art) that are built using similar quick connects from the needle valves for the co2 line so I assume it is a air/gas tight seal? (tubing fits, but the play/wiggle-room in the line at the connection isn't the most reassuring air-tight, snug fit feeling)
Quick Connect 1/4 in. x 1/2 in. Plastic MIP Adaptor-PL-3007 - The Home Depot

I can probably find nylon/plastic 3/16" ID barb fittings online, but this was a same day build and use reactor. I can simply unthread either the brass or quick connect once proper size nylon barbed fittings come in, but wanted to know how true co2/brass chemical reaction was. Looks like I will play it safe than sorry, to avoid any worries. I will order some nylon barbs.

Still open to hear others out.

EDIT: Just wanted to add that my pH is 7.4 before co2 injection, then the 1 point pH drop down to 6.4 for 30 ppm dissolved co2.

 

A couple points that may mean a lot?
One is how much contact there is between our water and all kinds of metal. From the time it is pumped to several weeks later it almost all runs in various metals. Iron, steel, copper, and brass as common. In some areas lead also! Your faucets are quite likely to be brass even if plastic on the outside.
Two is that the major regulators are brass as it is very resistant to corrosion. Good enough for the navy and ships.
These two items, combined with the amount of water changing I do, leaves me with little worry about my fish. I do not keep sensitive items, though.
If it were a major concern, I would move to plastic that is available from these:
Arkansas plastics (Arkplas?)
US plastics?
Grainger?
Johnstone supply?
A well stocked local wholesale place should have all the common items that we use.

 

so if the reaction is mainly a problem when running water through brass or copper piping, is there any potential danger in running co2 through copper piping without water present?