Asset Fee charges and Fiduciary Liability

[Guest erisa15]

We have a fund that will have various investors, some of which will be retirement plans. The fees charged will vary depending upon the size of the investment by the plan, individual or other entity. I have been told that charging the different fees to the different groups could cause the fund to become a fiduciary to the underlying retirement plans. Does this ring a bell with anyone. I am researching DOL opinion letters and PTEs to see if the issue has been addressed. Thanks in advance for your thoughts.