Not only does water block radio waves in general; water happens to be particularly good at absorbing radio waves near 2.4 GHz in frequency, which is what most WiFi uses. 2.4 GHz is also the band used by your microwave oven; microwave ovens work mostly by heating water that's in the food. This should tell you something! It is not because water molecules "resonate" at this frequency; that's a myth. Nevertheless the water gets hot because it's absorbing the RF energy. Your WiFi will not cause the aquarium to heat up (the power is far too low for that) but the water will absorb the radio waves just the same.
Therefore: if you put the aquarium near the WiFi router, it will create a "shadow" in the radiated energy, just as if you replaced the router with a light bulb and the aquarium with an opaque block. The bigger the aquarium, or the closer it is to the router, the bigger the shadow will be. Now radio waves do "diffract" somewhat around obstacles, so you won't likely see a complete absence of signal in the "shadow" area. But it will be reduced.
Note that this applies to everything else that uses WiFi, not just the router. Your computer has to send back to the router, after all. So if the aquarium is near the computer and between the computer and the router... same issue. (As we hams say: "If you can't hear 'em, you can't work 'em.")
On the other hand, if the aquarium, router, and computer are in locations such that they form a triangle, i.e. the aquarium is not between the other two, it's unlikely to affect anything. If the aquarium is between the other two but they're not close to either, again, the effect is likely to be quite small due to diffraction.
Incidentally: The reason the 2.4 GHz band is available for unlicensed use, like WiFi and microwave ovens, is exactly this: The water vapor (humidity) in the atmosphere absorbs so much energy that this band is nowhere near as useful for long-distance comms as other microwave bands. And the fact that WiFi doesn't go long distances well is also what makes it possible for so many people to use the same channels over and over, just by being a short distance from each other. Those long-distance WiFi contests that achieve tens of miles? They are done in the desert outside Las Vegas, where the air is very dry. Of course, it also helps that they use huge dish antennas and there are very few or no other WiFi signals around.