Along with your research you may include one treatment in acuaculture experiment of phytoplankton which is essential for dissolved oxygen in acua culture pond water and also supply nitrogen and phosphorus for nutrition of protein to fish which is essential for growth and development.

I would suggest a mixed effects model (cage is random effect). Linear mixed effects models usually do a pretty good job with livestock growth, I'm unsure on fish growth but I would assume it's somewhat linear-esque. In that case, look into lmer R package. You also need to check the effect of tank as your cage is inside tank in your study design.

More generally, you would want to average the weights of animals in a tank (can do this in excel or in R) so that tank weight is your outcome variable. If your animals are mixed genders but you have a gender proportion I would see if that was also predictive.

Excel columns need to include Tank, Date and Weight, and Gender if recorded. I usually recode my dates to be Day of Trial or Week of Trial (again, either in R or excel) and may run as a factor if the study is very long or run by phase (recode 0-21 as starter, etc. etc., however appropriate for your species).

I hope this helps. Feel free to message if this is unclear or you think I can help on something further.

I think you could use a repeated measurement ANOVA, it is know for psicology studies since it is often to use the same test subject in different threatments or over time, but it fits your study exactly by the same reasons. see https://www.google.com/amp/s/www.r-statistics.com/2010/04/repeated-measures-anova-with-r-tutorials/amp/