Monitoring fish growth
This is a good experiment!


3 types of food: ok
9 tanks: ok
Apply different treatment randomly - no!  You are right - no way to tell with random.  It is like comparing McDonalds to vegan diet - you have to stick with your diet to be able to tell a difference in the end.  So Tank 1,2 and 3 get food A.  Tank 4,5,6 get food B.  Tank 7,8,9 get food C.  Each tank gets only that food type.
You need to add the same weight of food to each tank.  More food = more weight.  I am not sure how much is a reasonable amount; it depends on your fish.  Get out what looks like a reasonable amount and weigh it.  That is how much food each tank gets each day.  You could weigh a bunch of quantities and set them aside - maybe in an old egg carton?  That will make it easier to feed the fish each day.  
Have the same number of fish in each tank. 3?   Ideally fish are same species and all about the same size to start because different size fish might not all gain weight at the same rate.  I am picturing 11-cent goldfish.  3 per tank makes 9 / food type and 27 in all which is good.
You can gauge growth by fish weight.  When it is time to weigh the fish, fill a bowl with aquarium water, put it on the balance and zero the balance.  Now you will get the weight of whatever you add to the cup because balance is ignoring weight of cup and water.  Sweep a fish out of the tank into the bowl and weigh it.  Then dump it back in and repeat with next fish.


If you can tell the fish apart each one can be weighed separately and I think should also be named with a name that starts with the letter of its tank because that is fun.  For example TankB4 will contain Burr, Black Beauty and Buzz.  Tank C7 will have Clyde, Caesar and Coco.  


Keep track in your lab notebook of each fish and its weight
Take a photo of each one when you start and one when you are done.  This will make a great poster.


You can weigh fish every day or maybe every 3 days if you think handling them is too stressful.
You can track weight for each fish, average weight for each tank, and average weight for all 9 fish (3 fish / tank x 3 tanks / food type) over time.  You will graph each of these.  You can use Excel or do by hand.



If a fish or 2 die your data will still be good. 

Good luck!  I hope you will come back to this idea and post an image of Aloysius, Angel and Aesop, the fish from tank A1.