I have two items in my armory that are showing as Teir 0, when I try to use them in a battle, they do nothing.

Magma Shard Dark Light Potion

I assume this is a bug?

I also tried using a Teir 1 Water Shard and nothing happens when I use it.

These items showing up in the armoury is an error. They are equipable items. When they are in your inventory you should have the option to 'equip to [pet name]' When you do that they should work when you use them in battle. Additionally, if you have them equipped to your pet as well as in your battle set through your armoury, when you use it in battle you will use the items from your equipment, not the item in your set.

When these items were first made equipable, they never properly got coded out of the armoury I guess. This really should be fixed. A lot of items like these that were made either into equipable items, or had their weapon status removed, are still showing up in the armoury. Thanks to the Troll, a lot more of them are being discovered, but so far, nothing has been done about any of them. :(