Answer:
1/6 ton of fish is fed during the feeding at night.
Step-by-step explanation:
The fish fed in the morning is
tons, and during after noon the fish fed is
more than in the morning; or

and let us call the tons of fish fed at night
, then if the total tons of wish fed during the is
, we have

Let us add the numerators on the left side and we get:

and subtract
from both side to isolate 

and since
, we have

the common denominator of the fractions is 6; therefore, we rewrite the equation as

and subtract the numerators to get:

which the number of tons fed during the feeding at night.