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I have a bunch of seashells I bought at the craftstore a long time ago and I was just curious if you can put them in the tank without screwing any of the parameters up or if it's just safer to leave them out.
It depends on what kind of fish you have. Livebearers like mollies prefer hard alkaline water, so seashells won't do any harm.
The vinegar test means dropping a bit of vinegar on a rock or shell to see it it reacts. If it fizzes, acidic water will also react with it, relaesing minerals, hardening the water, and raising pH.
If the pH is 8.0, the seashells would help buffer the pH. Crushed coral substrate does the same thing, I use it in my african tanks. Seashells alone won't keep the pH from dropping from biological activity over the long run. I add baking soda and epsom salt to my water to raise and stabilize the pH between partials. The Tanganykian shell-dweller cichlids actually spawn in snail shells, and would be a good choice for a small (arouund to gal) tank with hard, alkaline water. I found out through experience it's easier to raise and maintain high pH than to lower it.