A few issues in this thread on which I'd like to comment.
First, I do not recommend turning off a filter in any aquarium. I only do this when performing water changes, so it is maybe 20 minutes at the most. The bacteria in the filter media need oxygen to live, and they obtain oxygen from the water passing through the media. When this stops, the bacteria will start to die off after a couple hours. Then when the filter is on again, this dead bacteria enters the tank. Not good. Which is why after a power outage of a few hours or longer, you should always rinse out the canister filter before the power comes back on.
Second issue, the duration of light should be consistent each day. Plants and fish are affected by the duration of light and dark. A timer available in hardware stores for table lamps works well; the light should come on when there is light in the room, and go off when there is light in the room--this to avoid startling and stressing the fish. But the light duratio should be the same each day. I suggest 10-12 hours max; sometimes if algae is an issue, reducing the duration by an hour or two hours works.
Now to the plant issues. Low, medium and high light plants is a very subjective rating and not always reliable. As I have proven and written of many times, plants require considerably less light than many would have you believe. However, there is still difference from plant to plant, and just as there are plants like crypts,
Anubias,
Java Fern, moss that manage fine with fairly little light, there are those that need more. Red leaf plants do need more light than green. To explain why needs some background on light.
Plants primarily use red and blue light in photosynthesis. They appear green because they reflect green light, since they do not require it. So red leaf plants are reflecting red light, and since they need red light, they need more of it because they are reflecting much of it.
You can try the Alternathera reinckii, it may or may not last. But before you consider increasing the light intensity to provide more for it, remember that doing so creates a totally new "balance" between light and nutrients. In other words, more light will require more nutrients in balance, or algae will be everywhere in the light. This is not something I recommend unless you are prepared for the consequences.
As for intensity and duration, plants require adequate light and this means intensity and duration. Increasing one without the other achieves nothing--except more algae. And the nutrients have to balance intensity and duration, or the plants can't use it anyway.
You mentioned substrate tabs for Rotala; this won't be of much help. Stem plants assimilate nutrients via the roots and also the leaves. Substrate nutrients have little impact on stem plants; a liquid fert like Flourish Comprehensive will achieve better results.
Last comment, not all plants will grow in the same aquarium together. In nature, there is usually only one or two species of plant in any given section of stream. Only in our aquaria do we combine so many different species of plants. The fact that they all usually grow well together is an amazing bit of nature. But some plants produce chemicals that can affect other plants, this is a defense mechanism called allelopathy. This is a science in itself, and I only mention it.
Byron.
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