When the fish-tank water has a pH of 8.0, the hydronium ion concentration is 1.0 * 10-8 mole per liter. What is the hydronium ion concentration when the water has a pH of 7.0?

You must know and use the formula for pH.

pH = - log [H3O+], where [H3O+] is the molar concentration of hydronium ion.

So, when pH is 8.0 = > 8.0 = - log [H3O+] and you can use antilogarithm (the inverse function of logarithm) to find [H3O+], in this way:

[H3O+] = 10^-8 = 1 * 10 ^-8 M

When, pH = 7.0 = >

7.0 = - log [H3O+] = > [H3O+] = 1 * 10^ - 7 M

Answer: 1*10^-7 mole / liter