Essjay has answered your last question in post #21 and I agree totally. To your other questions...
Is it possible to get away with having high light but not dosing co2? Would I have an algae outbreak? Would it kill my plants off?
Another explanation. Aquatic plants need light and nutrients and in an aquarium these have to be in balance for the plants (species and numbers) or algae will become problematical. Plants grow via photosynthesis; if photosynthesis is not able to occur, the plant dies. Light drives photosynthesis, but it must be of sufficient intensity to do this, and different plant species have differing needs when it comes to intensity. Generally, slower-growing plants need less light [just like shade plants in the garden tend to be slower growing than sun plants] while faster-growing plants need brighter light. At the same time the spectrum factors in; aquatic plants need red and blue light to drive photosynthesis, and of these, red is the more important. This is why actinic light and similar marine tank lighting that is high in blue with very little red does not work over freshwater, and LED light tends to be higher in blue though there are exceptions. Adding green to the red and blue has been proven to aid plants; while not needed for photosynthesis, it seems likely that the green adds intensity which does benefit. Plants that are green leaf are reflecting green light, just as red leaf plants reflect red light, so they need more of this light. Also, the sun at mid day is high in red, blue and green. A Kelvin of somewhere in the 5000K to 7000K range seems to provide this; "daylight" tubes/bulbs with 6500K are ideal.
Along wiith the light, nutrients must be available, all 17 of them. Light without sufficient nutrients, or excess nutrients without sufficient light, will advantage algae. Algae is not so fussy as higher plants when it comes to any of this. Over the years, I have seen algae increase with too much light (which can be as subtle as the additional daylight entering the room in summer), too little light, too much fertilizer, and not enough fertilizer. It's all about the balance.
So, having bright light with insufficient nutrients is not going to benefit the plants. Obviously, all of this is general, as we have no absolute specifics; the plant species and numbers factor in, the duration somewhat--longer duration does not make up for insufficient intensity, or the reverse, but on the borderline the duration can matter. I have my tank lights on for 8 hours, but only 7 in the 70g, and this is as long as I can manage or algae increases. If my lighting was brighter, I would not be able to go this long, but it also might be too bright period.
CO2 (carbon) is one nutrient, and a macro-nutrient so it is important. As soon as you add diffused CO2, you must have the light intensity to drive photosynthesis and the other nutrients must be available to the plants, or the additional CO2 will just feed algae. This is the case with light and any of the nutrients. Which leads me to the next question.
I was just thinking about dosing co2, iron, and potassium. What do Flourish Tabs actually do, if they don't dissolve? Have you had good experience using them? Also, is dry-dosing some what harmful to the fish?
Dosing CO2, iron and potassium is not going to benefit plants [beyond a certain point, which again depends upon the plants and other available nutrients which includes the GH (primarily calcium and magnesium)] and may, subject to the fore-going, cause algae issues. All nutrients that plants require can be supplied by fish foods and water changes; but these alone may or may not be sufficient for the plant species and numbers, and then the light intensity factors in too.
If supplementation is required, you have to be careful here too. Plants require nutrients in balance with each other. Too much of one nutrient can cause plants to shut down assimilation of another nutrient. So it is possible to kill plants with too much fertilization, just as with too much light; or with insufficient of either (which is much the same thing, only looked at in reverse).
Nutrients must get into the water in order for plants to assimilate them via roots and leaves. Liquid fertilizers are effective for this, but if you happen to overdose, then algae can take advantage. Substrate tabs can benefit because they do not break down the same way to enter the water column, or so Seachem says of their Flourish Tabs. I cannot explain how this works, but I will honestly say it does seem to be a valid claim. I changed to more substrate tabs (more often) and reduced the liquid fertilizer, and I saw a noticeable decline in algae issues while the plants did respond as well or even better (my very soft water being low in calcium was a good test, and the tabs did solve this without my having to add calcium and magnesium in liquid form).
Flourish Tabs, like the
Flourish Comprehensive Supplement for the Planted Aquarium both contain all required nutrients [except oxygen, hydrogen and carbon] and in a relative balance according to what botanists say the plants need. Some of the macro nutrients are minimal, because Seachem understand most have these in the water (GH being higher) along with fish foods. Carbon (CO2) will naturally occur, and usually sufficient to balance light if it is not too intense. The other nutrients are fairly easy to add via fertilizers of some sort, and the "comprehensive" ones are best in this situation. I have some planted tanks that need no fertilization, others that do, all depending upon the plant species and numbers, fish load, and light.
In high tech tanks you need more nutrients to balance the more intense light, and this is where dry fertilizers are usually used as they are less expensive and easier to dose in larger volume. The so-called Estimated Index (EI) method became popular several years back; this involves dosing excess nutrients, then doing a weekly major water change to remove any nutrient not utilized. This certainly can work, especially in high-tech systems. But it comes at a cost to fish.
Every substance added to the tank water gets inside the fish. Water is continually entering fish via osmosis through every cell, and via the gills. Fish don't drink because they take in the water this way. So every substance in the water, from water conditioners to plant fertilizers to medications and other additives...all of these get in the fish's bloodstream and internal organs. Obviously in its habitat the fish has none of these. While some of these may kill certain fish, even those that do not are still impacting the homeostasis and metabolism and physiology. So, in the interest of providing better care for healthier fish, we should do everything we can to reduce additives and only use those absolutely essential. This is why I do not recommend EI, or CO2, or bright light.
I'll end this "essay" with one simple example. I have a shoal of
Paracheirodon simulans (the false or green neon, very closely related to the cardinal tetra) in my 40g flooded Amazon forest tank, along with a shoal of
Nannostomus eques, a shoal of
Hyphessobrycon amandae, and a pair of
Characidium fasciatum. All these fish are wild caught. Their habitat waters had zero GH and KH, very low TDS (total dissolved solids, another topic), and a pH below 5 depending upon the locale (these fish are not sympatric in most cases but come from very similar waters, including blackwater). What can we assume will occur in these fish if we start dumping fertilizers into the water? One reason I have had some of these individual fish for 8 years and others less (so far) is because my GH and KH are zero, my TDS is extremely low, and the pH is at or below 5. If I started adding just one mineral, calcium, to "benefit" the plants, I risk calcium blockage of the kidneys; this has been proven to occur with cardinal tetras in harder water and it is unlikely that other species from similar habitats will have evolved much differently.
