Re: Test kits. You can buy kits to tests the nitrogen cycle which I shall explain briefly:
All animals produce amonia. Because fish produce it, their bodies have higher amonia levels than the environment around them. Through a process called osmosis, amonia molecules move freely through fish membranes until both the fish and the environment have the same levels. Bacteria in the environment consume the amonia, and so the process continues with the fish continually getting rid of amonia = happy fish
If your fish tank doesn't have enough bacteria to consume the amonia, then the fish dies of amonia poisoning. Nitr
ite is similar.
Nitr
ate is the end product of the nitrogen cycle. In a natural ecosystem, nitrate is absorbed only by plants. Most fishkeepers prefer to do a partial water change to dilute the nitrates and other foreign substances *
I consider nitrate test kits more valuable, because nitrates are an ongoing concern. In contrast, your amonia/nitrite levels are only going to rise if you kill or remove the bacteria in your tank. You can do this in two ways: 1) Poison the bacteria, or 2) remove the filter containing the colonies of bacteria.
For this reason, you will only want to test for amonia & nitrite when your filter is new, or you added some chemicals to the water. The petstore can probably test a water sample for you, for less than the cost of a kit.
Re: Your plants. There are different species of Pondweed.
Although I wouldn't use it (I already mentioned a preference for Duckweed), your Pondweed will grow very quickly and absorb lots of nitrates. You will want to remove the plants when they grow too big, cut off the old stems, and return the new growth to your tank. It probably looks dark with many leaves atm, but when growing quicker (i.e. in a warm tank under a tungsten bulb) it will look lighter and be less dense.
Re: Substrate - for colonies of bacteria.
Do not use coral: It will disolve calcium into the water making it very hard. This is fine for marine fish, but with the exception of some African species, it will result in undesireable water conditions for freshwater fish. Tetras prefer softer water. You can test your water for hardness (dH) but it's not normally required.
Tetras won't care what the substrate is, and it deprives them of swimming space anyway. Larger pebbles are easier to clean. Most fish prefer a dark substrate that reflects least light.
Substrate feeders (i.e. catfish, many cyprinids, loaches, eels &c.)
will care about the substrate because they live on - and sometimes in - it! Substrates with sharp edges can damage their soft mouths, bodies, barbels, or other extensions. They like to move substrate around and uproot your plants. Because your fish would be small, they need a substrate made of smaller particles. I would suggest using
very small rounded stones, or sand, but it's harder to clean.
You could try unusual substrates such as wood or zeolite. Zeolite is a porous white ore that absorbs amonia, but you will need to remove the granules from your tank before you can release the amonia trapped in them. Quite useful stuff
Some woods and sands release toxins into the water so you cannot take random stuff from the garden.
Whatever substrate used will need cleaning to remove dust: stick it in a clean bucket and run it under tapwater until the water runs clear. I suggest buying a new bucket just for fishkeeping, because you don't want the residue of any household cleaning stuff getting into your small ecosystem, and you will often need a bucket
They might think your are nuts, but ask the petshop for a tiny sample of gravel from a tank (containing only healthy fish) because their gravel will be thriving with the bacteria you're hoping to cultivate.
* There are lots of other ways to remove amonia, nitrite, & nitrate from water. Your tapwater may also contain an amount of these substances - but you probably use the same tapwater as the local petshop so the fish are used to it
