I used a 10 lb bag of Florida crushed coral on one of my tanks, and it just seemed to me like white, chalky gravel that was rough feeling, kind of like sandpaper. Quite different from sharp, thin, pointy slivers of eggshells! Putting eggshells in the filter in a sock would be fine, but as a substrate it is completely unsuitable. I dunno...the difference seems obvious to me.
I've read reports of puffers hurting their teeth on MTS. I don't think they usually go after them if there are other snails available. I think it's fine if a few get in there, but don't intentionally feed them to your puffers...would be my advice. Not sure about Amazon puffers being an exception, you should try asking Jeni (pufferpunk) at The Puffer Forum, if you are a member there. Or you could try asking the WWM crew a question at Wet Web Media. I know Neale Monks (a crew member there) has written lots of articles about FW puffers for websites and magazines.
I was just talking about the difference between culinary sea salt and marine salt made for the aquarium hobby. Even though Oceanic sea salt is called "sea salt" I would consider it marine salt...I am sure the ingredients differ from one salt mix to another, but I don't think it matters in this case very much, you are using a minuscule amount.
Even though they have been shown to be perfectly safe, there are also anti-caking agents in culinary salt, I believe yellow prussiate of soda is what is used in Morton sea salt. I'm sure marine salt for the aquarium hobby also has anti-caking agents, they are just not listed as such.
Bottom line, sea salt for culinary use is 100% safe to use in aquariums, and is a lot more economical and convenient if you are using small amounts. Does it provide the same benefits as marine salt made for aquariums? I doubt it, I think there is a lot more to marine salt, judging from the 80 elements listed in that article I linked to. Luckily, you have the Oceanic salt already. Just seal it tightly and keep it in a dry and cool place so it doesn't cake too much.
That last recipe sounds fine, just be sure to test your water so you know how much it is spiking your pH, KH, etc. and only use the buffered water for water change water, not top offs.
