This is much better done with a visual/drawing but I will do my best this way:

6/8 x 7

6/8 + 6/8 + 6/8 + 6/8 + 6/8 + 6/8 + 6/8

take the final 6/8 and break it into 6 equal pieces (or 6 1/8 pieces) give 1/8 to each of the previous 6 buckets making them all 7/8

(now 7/8 x 6) ( or 7/8 + 7/8 + 7/8 + 7/8 + 7/8 + 7/8)

divide up the final bucket of 7/8 into 7 1/8 pieces like we did before

then you can “borrow” 5/8 to fill the previous 5 buckets, making them full gallons and you will have 2/8 left over

(8/8 = a full bucket which = one gallon)

1 gallon + 1 gallon + 1 gallon + 1 gallon + 1 gallon + 2/8 of a gallon

So she will need 5 and 2/8 gallons to fill her fish tank.