science fair

Another easy one to do would be nesting site preference in a certain species. Get various pairs of a species and set the tanks up in the same way. Decorate them  with different types of nesting sites (ex: cave, stone, flat surface, various hole sizes in terra cotta) and see if there is a certain preferred type of nesting site for that species.

Dare I say convicts are a good species to use!

How about a feeding frequency or food type and growth experiment.

Otherwise, I'd do a simple waterchange frequency experiment like stated above and see what impact water changes have on growth rate.

You could look at nitrate build up over your water cycle and nitrite, amonia when you cycle a tank.  See what happens when a water change occurs.  You could see who is the dirtiest fish(Oscar, convict, frontosa, etc.).  Also you could get a good idea of when you absolutely NEED a water change.

You'd need to keep the stock constant in the tanks you want to measure. 

A couple years ago I had to do my project. At that point in time I did growth rate for plants with different types of waters used. I would think you need to set up 1 fish per tank/container to control the variables as much as possible. I would do 5g containers or clear shoe boxes, and just see what effects different types of water changes have on the growth of the fish.

It would be best to use an easy to spawn species such as a convict, and be able to keep the fry in 5- 10 gall on tanks. Pull all the fry at 1 week past free swimming after being fed a diet of Live BBS.

I agree.  You would keep temperature, feeding, amount of light/dark and other factors constant.

I vote for the effect of water changes on the growth rate of fry. Tank A- no changes, Tank B - 1 change a week. Tank C- 3 week, etc... Fry can be measured once a week.

My daughter will be doing a science project this semester in her 8th grade class. 

Anyone have a good idea?  Simple is okay.  She wants to do something with fish and I am fooling around with a growth rate experiment -

Controlled tanks with different food, or temperature or cleanliness, or stocking, etc.

It is a pretty quick deal - she needs to be done in a couple of months.