you can only research one level of a category at a time, however you may research more than one category at a time, if i remember correctly
and you should be able to have 20 fish per tank, i almost always have that
check to make sure there are no dead fish in the tanks, they take up a space, same as a live fish
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Why is it that if someone tells you that there are 1 billion stars in the universe you will believe them,
but if they tell you a wall has wet paint you will have to touch it to be sure?