Good question! I've also wondered about this whenever I have seen fish being sold in bags or small glass bowls at large discount stores or maybe at a county fair (where you can "win a fish") - it seems like they have put food coloring in the water to make it more attractive. I think we need a professional fish biologist/aquarist to give us the final answer to this, but I tried to find an answer for you in the meanwhile. If my answer is incorrect, I apologize. I think most serious aquarists and also veterinarians would recommend against adding food coloring, even if we don't know for sure whether it is harmful. I agree with this.

Companies that make the common food coloring that you would buy in grocery stores have done tests to address the question - these dyes, when used at the recommended concentrations, do not seem to have any harm. They are water soluble and don't seem to affect the chemistry of the water. Fish are sensitive to the levels of dissolved gases in the water (especially oxygen), nitrogen levels, and pH and the small amount of dye doesn't seem to matter. These studies were all conducted over a short term, however. I could not find any information that related to long term affects. Sometimes, we may not see an effect short term but over time, that could change. If water quality was affected slightly, it might take longer to have an effect on the fish. It is possible that the chemicals in the dye, technically "pollutants" would have a harmful effect on (be toxic to) the fish (or its offspring) if they built up over time. Also, changing the water color might affect how well the fish could see their food, and this would have a longer term effect.

There are some special food colorings that are oil based - they are used for fancy egg coloring and specialty pastries, for example. I did not ind anything to indicate that these had been tested on fish, but would guess that they might be harmful even in the short term - too much oil in the water could affect the water quality by preventing gas exchange. I bet you know why this is. You probably know about oil spills and the harmful effects they can have on wildlife. If you take a glass of water and dump a little bit of cooking oil on top, it is easy to see why having oils go into the water could be bad thing.

Food coloring that is ok for humans to eat, such as many vegetable-based dyes, should not be harmful to fish.However, as with most things in life, too much food coloring would not be so good for fish. It only takes a few drops to change the water color in a 10 gallon tank. And depending on the type of filter system the tank has, the color change may not last very long. I would like to add that there really isn't any reason to add dyes to your fish tank unless it's included as part of a medication/cleaning product. If this is the case, then you should always follow the product directions closely! If you just want the tank to look prettier, then a colorful backdrop for your tank will look nicer, and last longer. I hope this answers your question. Keep em coming!

According to a company that makes food coloring, it wont. Check out
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As long as you are using just a few drops for the whole tank, it looks like it will be fine.

You are right that water quality is important to fish. They breathe across their gills, which means that their gills are very delicate and can be damaged by toxins (bad things) in the water. Tap water itself is usually dangerous for fish because of the high level of chlorine used to make it safe to drink. Before using tap water in your fish tank, let the water sit in an open tank or bowl for a few days. This allows the chlorine in the water to leave as chlorine gas.

Do you think the food color changes how fish see whats inside and outside their tank?