I can promise you, your subtank is not Leaking due to sitting on any given Mod.
Bad Seal ring - Inspect upper/lower o-rings for glass section. Inspect Glass for any missing pieces(edge chips)
Check O-ring under Coil Head and verify Head is installed securely.
As above, Seals cause leaks most often, second to improper assembly. Heads can on occasion be an issue.
Tanks need to stay under negative pressure to function properly.
*avoid laying tanks on their side for long periods*