You can use program Growth II - it is very easy and with a useful guide to it for the types of growth curves. This is the site where you can buy or see demo version http://www.pisces-conservation.com/softgrowth.html
I work on marine species but I think there are no big differences, to model growth, with freshwater species. If you have age and length you can trivially try to adapt this data to any growth model using Excel and see which model responds best (best fits) to your experimental data. In addition, FAO and other organizations provide many free software packages for this purpose.
The growth modeling depends on the number of data or population size. Dear friend I suggest you for simply length weight calculation and got information on the growth status of fish.