The smeltery is alloying both Brass (copper and zinc) and Aquarium (copper, zinc and prismarine) at the same time

If crashed, link to crash report (use a site such as pastebin):

Versions:

• Minecraft: 1.12.2
• Forge: 14.23.5.2847
• Mantle: 1.3.3.55
• Tinkers Construct: 2.13.0.183

Can it be reproduced with just Tinkers Construct? If not, list the other mods required to reproduce the issue.
Base Metals: 2.5.0-rc2.332 https://www.curseforge.com/minecraft/mc-mods/base-metals/

We don't add an alloy for aquarium, go yell at the mod author who added an alloy with the same materials as a vanilla alloy. I'd assume based on your report it's base metals, they should make aquarium require brass.