Why if I put wireless access point near water (fish tank) will degrade its signal?

I am reading this article: http://gnswifi.com/faq.htm

And for this question:

What can I do if I am having wireless connection problems?

It says:

Signal strength drop or fluctuation are common causes of RF interference.

• Change the channel on your access point or wireless router. Use only channels 1, 6 or 11 for non-overlapping channels.
• Change the location of your wireless products. Subtle changes (2-3 feet) can make a big difference. Do not put the access point or wireless router in a cabinet or enclosure.
• 2.4GHz phones, X-10, and Bluetooth devices will interfere with your wireless network. Change the location of the base for your phone, or downgrade to 900MHz phones, or upgrade to 5.8GHz phones.
• The wireless signal will degrade (or die completely) when going through brick (fireplace), metal (file cabinet), steel, lead, mirrors, water (fish tank), large appliances, glass, etc.

If your wireless connection is only dropping during large file transfers or when a large number of wireless clients are connecting, change the preamble on all wireless devices to short.

My question is: Why if I put wireless access point near water (fish tank) will degrade its signal?

Water can pose a lot of resistance for RF signals. Here's a technical document the explains this is great detail, which you might find interesting since it's relevant:

  Underwater Radio Communication, by Lloyd Butler VK5BR (1987)
  https://web.archive.org/web/20110525004341/http://www.qsl.net/vk5br/UwaterComms.htm

These two quotes from that article address your question:

• "Water in its pure form is an insulator, but as found in its natural state, it contains dissolved salts and other matter which makes it a partial conductor. The higher its conductivity, the greater the the attenuation of radio signals which pass through it."

• "Attenuation of radio waves in water (and, in fact, in any conducting medium) increases both with increase in conductivity and increase in frequency."

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.

If I recall correct from my college years. This happens because of two attributes of water.. First as it has mass it is a natural obstacle and so the RF cannot pass through it as if it was air. but the most interesting thing is that its reflective nature doesn't help the signal, because of its not still surface. This happens because lets say you send the following data packet "101010" not all values will reflect the same way so you might get "111000" and the protocol has to wait for the packet a second time and so on. (this is a true problem in long distance communication that's why we use differential antennas I think that this applies to WiFi RF too) I hope I helped.