The point is to allow filling the tank only with lava. It still accepts water though....

@Override
	public int fill(ForgeDirection from, FluidStack resource, boolean doFill) {

		int df = this.tank.fill(resource, doFill);
	    if ((df > 0) && (doFill) && (resource.getFluid().equals(FluidRegistry.LAVA)))
	    {
	      this.worldObj.markBlockForUpdate(this.xCoord, this.yCoord, this.zCoord);
	      markDirty();
	    }
	    return df;
	}