I've had great results with blackwater extract by doubling the recommended dilution. You can also fill a large pan with water and a few handfuls of oak leaves and bring this to a steady boil for 20-30 minutes. After the water has cooled to room temp, you should have a tea. Depending on how 'thick' the tea is, it may need to be diluted a bit with dechlorinated water before being used. Lately, I've found myself just using standard treated/aged water for my tadpoles and keep pieces of oak leaves in the rearing vessels to help keep the tannins there. The tads also like munching on the leaves and I have noticed a drop in SLS (spindly leg) occurances with my vents using this method. Hope this helps.
It shouldn't be thick texture wise but should have a nice amber tint to it but not too dark....you should be able to easily see through the water when in a glass. If you plan on using oak leaves, you don't have to boil the water to extract the tannins but rather place 4-5 whole large oak leaves (brown and dried) in a gallon jug and fill with the water you plan to use for the tadpoles. After this sits for a few days to a week it will be noticeably tinted. I tend to skip all of this and just use oak leaves directly in the tadpole rearing vessels and let the tannins naturally leach into the water.
There are better brands of blackwater extract than tetra. The one made by Kent Marine always worked better for me at least with my fish. They make better quality water additives than tetra. The Tetra brand gave a more greenish tint to the water while the Kent produces a more brownish natural tint. It is very subjective but it seemed to work better for me. Check out http://www.drsfostersmith.com/produ...84&Ne=40000&R=2637&js_enabled=0&N=2004+112995
Why dont you just get some fresh bogwood from the Aquarium store for about $20, leave it in a bucket with some hot water (from the kettle), and youll have 5 gallons of the best quality BONAFIED black water, full of natural humic and tannic acids. Much better.
We use a 16 ounce cup of milled peat to about 8 quarts water in a large stock pot. We boil it for a little while and then let it simmer for a couple hours. Turn it off and let it cool overnight. This creates about 1 1/2 gallons of a really strong tadpole tea. We then take that tea and add it to aged tap water with Stress Coat in it until it is a light brown color. This works well for us.