Hi Malak.
Could you please let me know which recipe you’re referring to so I can give you a precise answer?
In general, though, you can use fish gelatin instead of silver gelatin sheets. The key to calculating the necessary amount is knowing the gelling strength, or “bloom,” of both gelatins, as it can vary.
Silver gelatin sheets usually have a bloom strength of around 160. If your fish gelatin has a similar bloom, you can directly substitute the quantity. However, if the bloom strength is different, you’ll need to adjust the amount according to the difference in gelling power.
For an accurate conversion, divide the bloom of the fish gelatin by the bloom of the silver sheets and adjust the quantity accordingly.
I hope this information is helpful.
Best regards.