Ahhhhhh, you have discovered one of the hidden benefitsd to using real wood in an aquarium. As it sits in the water, the natural tannins contained in the wood slowly leech out into the surrounding waters. Larger aquariums don't show the discoloration as quickly as smaller ones, due to the dilution factor. These tannins do slowly lower the pH of the surrounding water. The longer it is there, the lower it goes, basically, and doing small changes in a small tank allow the tannins to build up, where they would be removed quicker in a larger tank with corresponding larger water changesFish from the Amazon, or some other large water systems in rainforest areas would appreciate this type of water a lot. Neon tetras would thrive and breed readily in it, if they were not captive bred types that have slowly adjusted to higher pH levels. Abrupt changes to the lower pH ranges can be devastating to fish, even if they are from naturally low pH ranges. If you do decide to add fish, you will not be able to add a lot anyway, due to it only being a three gallon tank, but slowly accustom them to the water, just as you would adding them to a higher pH level tank. Or, adding new fish to an existing set up. Good activated carbon will help remove the tannis, and clear the water color if that bothers you. The tannins discoloration is just a basic warning to you of the water conditions. Many people store water in large plastic cans with wood to naturally lower the pH so they can do their water changes into similar set ups without disrupting the pH ranges they have worked so hard to get. Many killies, and wild caught imported fish, and even a lot of the captive bred fish you can get in shops will only breed in the low pH ranges, and this is one way of getting it without using a lot of chemicals that really don't work well, or from using expensive systems like RO units and ion exchange units. They may adapt to living in the higher pH ranges, but they will not thrive, and will not breed, and even if some of them do breed, the spawns will be very small, or the fry may not survive. With some fish like discus, it has been proven that harder water and higher pH ranges cause the egg skin to harden prematurly and before the male's sperm can fertilize the eggs, soo it looks like a good spawning, but the eggs just fungus over since they were not viable.

 

Fish like actually eat the driftwood, like some of the bristlenose plecs, and fish that grave on the wood for the small animals they eat from the surface of the wood, like the very small

plecs, will help keep the wood in better looking condition, but all wood placed in water will eventually rot and fall apart, and will need to be replaced. (except maybe something like ironwood from exotic places). Different woods used will make a difference also. Oak and some others are better for the aquarium, as things like pine has that natural pitch which doesn't do well in water. A friend of mine used to use applewood from his back yard, as he knew it was not contaminated with pesticides etc.  If you pick up wood, it needs to be treated to make sure it does not contain parasites, or other bad things, and the bark removed if present, as it will disintegrate and foul the water quickly.  Most wood will also float until it is saturated. So if you fetch your own, peel the bark, and soak it in a plastic trash can or similar, and use a med to kill parasites etc before using it in an aquarium.

 

I hope I got it all in here. If I missed a point, or you have other questions, ask away.