If you use various FIAT currencies in your account, there will be error messages for those that are not your main account currency.

You can view (and change) your main account currency on the Account Settings page.

The second FIAT currency will be treated as a foreign currency in our system and will be assigned a cost basis of 0 if there is no matching import. 

Please enter all second FIAT deposits as 'Income' since this transaction type is taken into account for the purchase pool and has a Fiat cost basis.

If you live in a country that taxes income higher than capital gains, please use the following work around:

For example, if I have the following trade that showing a fiat deposit warning:

Buy: 0.15 BTC Sell: 40 USD

Convert it to your home FIAT (example: CAD) using the conversion rate at that time (which can easily be retrieved from CT by editing the transaction and using the "Sale value in CAD" value):

Buy: 0.15 BTC Sell: 50.78511838 CAD (the equivalent of $40 USD)

Please be aware that 

 by holding onto the other FIAT (i.e. CAD) for a little while after ‘earning it’ (as it was listed as income), and

before buying cryptos with it, you can lose or gain a little home FIAT (USD).