If he had 200 fish and sells 50, he is left with 200 - 50 = <<200-50=150>>150 fish
If a third of them get spoiled 1/3 * 150 = <<150*1/3=50>>50 fish get spoiled
The remaining number of fish is now 150 - 50 = <<150-50=100>>100
He then gets 200 more to make 100 + 200 = <<100+200=300>>300 fish
#### 300