Quicksilver Fountain 
Artifact
$3.56 #13382 0.57 22.9 years
At the beginning of each player's upkeep, that player puts a flood counter on target non-Island land they control of their choice. That land is an Island for as long as it has a flood counter on it.
At the beginning of each end step, if all lands on the battlefield are Islands, remove all flood counters from them.
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Cards similar to Quicksilver Fountain expected a card that isn't here?
Reports become test cases the matcher is graded against before an update ships. Delvefall isn't a chatbot guessing, it compares the printed rules text of every card with plain math, and examples like yours decide which differences should count.
Harbinger of the Seas74%
$1.56↓#3774↑1.41↑2.2 years↓
"Nonbasic lands are Islands."
Conversion72%
$0.24↓#26556↓1.18↑33.1 years↑
"All Mountains are Plains."
Xolatoyac, the Smiling Flood72%
$0.44↓#5015↑0.26↓2.8 years↓
"Whenever this card enters or attacks, put a flood counter on target land. That land is an Island in addition to its other types for as long as it has a flood counter on it."
Eluge, the Shoreless Sea72%
$5.08↑#3042↑0.74↑2.1 years↓
"Whenever this card enters or attacks, put a flood counter on target land. It's an Island in addition to its other types for as long as it has a flood counter on it."
+1 more matching line
Glaciers71%
$0.54↓#29115↓0.69↑31.3 years↑
"All Mountains are Plains."
Zhao, the Moon Slayer70%
$0.32↓#10209↑0.00↓0.8 years↓
"As long as this card has a conqueror counter on him, nonbasic lands are Mountains."