I expect that these questions have been answered before, but I couldn't find them in a search.

First, food. Does how long it lasts depend on how many fish are in the tank, or time? If I have, say, five fish in a tank does the food last longer before the level gets dangerous than if there are twenty? If I have the five fish and they all spawn and/or I add some eggs, bringing up the numbers to twenty, will the level start dropping faster?

Secondly, over the last couple of days I've found several mature dead for no apparent reason fish in my tanks. Shouldn't they live for ever in effect? Foods levels were still in the green. In one game I've got all the magic fish, plus all the levels and items from the store. In the other I've five MFs, again with all the levels and items from the store. All I can think of is that they become infected and die while I'm sleeping or at work. Although I do have the MF of Healing in both games. I'm finding that infections still occur quite often.

I've never had mature fish die (as long as they received proper care in an appropriate environment) unless they succumbed to disease. I have fishies whose ages are well over 6 thousand.

You could always perform a little experiment on the food (you didn't think I was going to tell you exactly how that works, did you?). Put a handful of fishies in one tank and fill another tank with fishies. Add food to both tanks until your fishies are fully fed, and then check them in a few hours. Compare results and you will have your answer.

ladycfii, are they real? j/k!

well, i had mine reach the cap LOL!

im gonna take LadyCFII's advise too

On a related note, I recently transferred a number of fish to the sale tank. I then made all six remaining fish pregnant, and sold those in the sale tank. On returning to the main tank, I discovered that one of the fish had become infected. I healed the fish with the fungal treatment, but found that she was no longer pregnant. Is this normal?

Yes, it's normal. The way the game was programmed, sickness and pregnancy are mutually exclusive statuses. Sick fish can't become pregnant, and if a pregnant fish gets sick, it will no longer be pregnant.