Number of dead fish:10x;

Sales of dead fish: 20 *10x = 200x;

Unit price of live fish: 30+x;

Quantity of live fish: 1000- 10x

Total fish sales: (30+x) (1000-10x)+200x.

=30000-300x+ 1000x- 10x^2=30000+900x- 10x^2

Other expenses: 400 times

So: q = (30000+900x-10x2)+(1000-10x) =-10x2+890x+31000.

(2) profit =Q- 1000*30-400x

=- 10X^2+490X+ 1000

=- 10(x^2-49x+24.5^2)+ 10*24.5^2+ 1000

=-(x-44.5)^2+7002.25

Therefore, the maximum profit can be obtained by selling after 44 days of stocking. The maximum profit is 7002.

(3) -(x-44.5)^2+7002.25≥6000

(x-44.5)^2≤ 1002.25

-3 1.6≤x-44.5≤+3 1.6

1 1≤x≤76