[Avodah] Drinking water after fish

[Akiva Blum]

The gemora at the end of the first perek of moed katan says to eat fish close to it's rotting. It also says to drink water with fish.
Tosafos says that nowadays we hold eating such fish is dangerous. Similarly, drinking water with fish. Tosafos answers (in the first answer) that maybe things have changed, like other refuos in shas which are no good nowadays.
R' Akiva Eiger SA (YD 116 on the Ramo in seif 5) paskens like this Tosafos not to drink water after fish.

Here's what I don't understand. RAE takes Tosafos as an absolute issur, not to drink water after fish. But surely Tosafos is simply pointing out that according to our present day understanding, we cannot drink water with fish. However, in a time where this belief isn't held, there would be no issue.

Was it believed in the time of RAE that there is some sort of danger of fish and water? Is anyone familiar with such a belief in the times of Tosafos, or any other time? What about today?

I have seen in a couple of modern halocho compilations the RAE quoted. But if we have no such belief of danger, this RAE should be irrelevant.

A GM

Akiva