First of all, ImageJ and Fiji is basically the same but Fiji is a more modern version with more advanced options and preinstalled plugins. It is therefore recommened to install Fiji and all further tutorials are performed with Fiji. However, if you were using ImageJ macros or analysis workflows in the past, it is possible that they will no longer function in Fiji. You can install both packages (ImageJ and Fiji) and use them both in parallel.
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You can download the packages here: https://imagej.net/software/fiji/downloads
