It looks like you have a couple of problems.

1. Your fish keep dieing. Ouch, nothing will help with the exception of checking in on them. The level of food research will keep your fish well feed for at least 24 hours, so if get that if you don't have it. Other then that, if you know you are going to be away from the computer for several days, pause the game.

2. Your selling price is affected by several things. Advertising, the amount of fish in the fish tank and the items you can buy in the store. Also Magic fish in the sell tank will bring more peeps in the store as well, which will help you sell them faster.

Does that help?
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The VI