Sorry, I just don't trust them. I did try 1 tank using these grommets, and it came apart(wasn't using the O rings). One of the grommets slid sideways, and the rest is history. It just doesn't seem that there is enough of the grommet making contact with the tube for good enough security. Perhaps if the grommet were thicker, I'd have more confidence. Just my .02 on these grommets. YMMV.
I have another idea for endcaps using the polypropylene tubing, waiting on UPS. Someone(can't remember) suggested to use these,
Q Slip Joint Washer Bag # 38809B by Danco Company. Before they are placed on the tube, they seem perfect. You need to put the carto into one of the the washers, push the washer into the tubing, and then push the other washer on. It works, it's extremely secure, and it looks good, but it's too tight to do a hot switch, and refilling needs to be done with a syringe.
So, I have a different size coming,
P Slip Joint Washer Bag # 38808B by Danco Company. I am hoping that this size will allow for easier refilling and hot switching of cartos.
It'll either be an epic fail or an epic win... I will post my findings.
I find that the grommets are much tighter once the carto is in the middle. Try it you'll see what I mean. I also use o-rings in the indendation of the grommets, 7/16 id 5/8 od are the right size for the o-rings.
For anyone interested in trying grommets make sure you get vinyl not rubber. You'll taste the rubber. Radio Shack sells vinyl grommets.