What Version of NeoTech are you using:
1.9.4-3.1.0
If possible, did you break and replace the block:
yes but doesnt contain the amount it should contain

Please Pastebin.com the crash Report and put link here:
No Crash

When upgrading an Iron Tank to a higher tier one, the fluid amount that it can hold stays at 8 Buckets. Rendering in Inventory is correct as well as displaying that it is only partially filled (for example 8 Buckets in the Iron Tank - 16 Buckets in the Gold Tank Rendering shows that the Gold Tank only holds 8 Buckets). In World rendering is same as Iron Tank for any higher Tiers and shows full after 8 Buckets