If a C-corporation owns another company's stock through brokerage firms (such as Fidelity and IB) where do you report those stocks owned in the balance sheet? And are there any other reporting requirements, tax wise?

Yes. If a C-corporation owns stocks through brokerage firms like Fidelity or Interactive Brokers (IB), those stocks are typically reported on the balance sheet under "Investments" or "Marketable Securities", depending on the classification.

1. If the stocks are actively traded and intended for short-term holding, they are classified as current assets under "Marketable Securities." 
2. If the corporation intends to hold them for the long term, they are classified as non-current assets under "Investments."
3. The valuation method depends on whether the corporation follows GAAP or IFRS accounting standards—typically, they are reported at fair market value. Your accountant on staff will know the accounting standards used for your firm.

Additional Tax Reporting Considerations 

1. Capital Gains Tax: If the corporation sells the stocks for a profit, it must report capital gains on its tax return.
2. Dividend Income: Any dividends received from these stocks must be reported as income on the corporation’s tax return.
3. Form 1120: C-corporations file Form 1120 with the IRS, where they report investment income and gains.
4. State Tax Considerations: Some states may have additional reporting requirements for investment holdings.