Aquatic Plant Biology and Technology
Meeting the needs of aquatic aquarium plants is much more about knowing biology than buying technology.
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Next, let's look at your substrate. Aquatic plants just don't grow well in an inert substrate like plain gravel. There are a number of different ways to go on this, but for first-time aquatic gardeners I suggest a good commercially available laterite intended for aquarium use. Laterite is easy to use, will not cause algae problems, and is not a danger to either plants or animals in the system.
Heat cables are a nice addition if you can afford them, but are certainly not a requirement for excellent growth. It is helpful to insulate the bottom of the aquarium to prevent heat loss from the substrate, which can be done inexpensively by seating the aquarium on a sheet of styrofoam.
While canisters are my filter of choice on larger planted aquariums, your filters are fine as long as you are replacing carbon dioxide (CO2) lost from the filters with supplemental carbon dioxide. There is no real need for the continuous use of carbon in a well-managed planted aquarium.
While your water is certainly on the hard side, it is not any harder than that of many successful aquatic gardeners in Europe. Water softening resins are not particularly useful in a planted aquarium. They just replace the hardness in the water with sodium, which is worse than the original hardness. As you discovered, small deionization (DI) units are quickly exhausted by very hard water.
In your situation, I would first try supplying supplemental carbon dioxide either with a do-it-yourself pressurized carbon dioxide system or, if you want to try it out in a less expensive way first, set up two or three pop bottle yeast reactors. Many aquatic plants are able to do quite well in even harder water as long as they have sufficient lighting and supplemental carbon dioxide. If you are not satisfied with the growth, I would suggest putting the money into a larger DI system. In the long run you will save money. Better still, you will not feel the need to skimp on water changes to conserve your DI filter.
Speaking of water changes, I'm sure your tap water phosphate and nitrate levels are lower than those in your aquarium. You must get those down before you increase the light levels or you will end up with algae problems. I recommend that you keep nitrate below 10 milligrams per liter (mg/L) in a moderately lit aquarium, and phosphate below 1 mg/L. That's for an aquarium without current algae problems. If you are trying to solve algae problems, I would aim for unmeasurable levels of both. In a brightly lit aquarium (3 watts per gallon or more) the lower readings are necessary to avoid algae problems.
The phosphate and nitrate levels are high enough that I'm quite sure your water changes are not keeping up with the organic input into the taquarium. I would recommend changing 25 to 30 percent twice weekly until the levels are reduced. Then, you can back off to 25 to 30 percent weekly. If the phosphate and nitrate levels remain low, you might be able to slowly reduce water changes, but I would never recommend going below 25 percent every other week.
It is almost always possible to "fix" an established aquarium so that you can get at least reasonable growth. However, for a great-looking aquarium, plus taking the high nitrate and phosphate levels into consideration, I would recommend draining the aquarium, resetting the substrate with a good commercial laterite, and refilling with water with lower nutrient levels. If you decide to work with your aquarium as it is now, you can improve the substrate by purchasing laterite in the ball form, These balls can be pushed down into an established substrate in areas where plants will grow. They cause minimal clouding when first pushed into the substrate, and that will settle out in a matter of hours.
Finally, we get to the plants themselves. I discovered in a conversation with you that most of the plants you had purchased were non-aquatics. This is the biggest problem of all! It doesn't matter how well your aquarium is set up if the plants you purchase are not meant for life under water — they will die. Some die slower than others, but in the end, the result is always the same. Buy a book with at least a fair selection of plant identification photos and take it with you whenever you shop for plants. If you can't find the plant in your book, don't buy it until you are sure it is an aquatic plant or amphibious plant. If it looks like a house plant, it probably is!
When you plant your aquarium, plan on covering at least 70 percent of the substrate with plants. At least 60 percent of this plant mass should be fast growers to handle the macro-nutrients in your tap water and those produced by your fish.
After about the first month, particularly if you are using supplemental carbon dioxide and at least moderate lighting, you will need to start fertilizing. Make sure you choose a complete trace element supplement that contains neither phosphate nor nitrate. These products vary in terms of dosing, but I would suggest starting with half the amount recommended by the manufacturer, and working up slowly based on the response of the aquatic plants.