To find out how many neon tetras the aquarium can house, we start by dividing the total volume of water in the aquarium by the amount of water each tetra requires.
The aquarium can hold \( 1 \times 10^5 \) gallons, and each neon tetra needs 1.25 gallons.
The calculation is as follows:
\[ \text{Number of tetras} = \frac{1 \times 10^5 \text{ gallons}}{1.25 \text{ gallons/tetra}} \]
Now, we can simplify this division:
\[ \text{Number of tetras} = 1 \times 10^5 \div 1.25 \]
Calculating \( 1 \div 1.25 \):
\[ 1 \div 1.25 = 0.8 \]
Now substituting back in:
\[ \text{Number of tetras} = 0.8 \times 10^5 \]
To express \( 0.8 \) in scientific notation, we can write it as \( 8.0 \times 10^{-1} \).
Now substituting this into our equation:
\[ \text{Number of tetras} = 8.0 \times 10^{-1} \times 10^{5} = 8.0 \times 10^{4} \]
Thus, the number of neon tetras the aquarium can house, expressed in scientific notation, is:
\[ \boxed{8.0 \times 10^{4}} \]