Here's why:
* Tin Goldfish's effect is a "cost" effect: It activates by sending itself to the graveyard as a cost. Costs are not considered part of the effect itself.
* Chain Summoning targets an effect: It chains to the activation of an effect.
* Chaining to a cost: Chain Summoning cannot target a cost effect, only the effect itself. Since the cost is not part of the effect, Chain Summoning cannot negate it.
Therefore, when you chain Chain Summoning to Tin Goldfish's effect, it will negate the special summon effect, but it will still have sent itself to the graveyard as a cost, allowing you to still special summon a monster from your hand.
Example:
1. You activate Tin Goldfish's effect to special summon a monster from your hand.
2. Your opponent chains Chain Summoning to Tin Goldfish's effect.
3. Chain Summoning resolves, negating the special summon effect.
4. Tin Goldfish's cost (sending itself to the graveyard) still resolves, allowing you to still special summon a monster from your hand.
This interaction was a commonly exploited loophole in older versions of Yu-Gi-Oh! and has since been addressed by many cards that negate "costs" instead of just effects.