The scientific basis behind CO2 injection is covered here.
The common definition of a high tech tank is one that uses carbon dioxide (CO2) injection. This means mixing carbon dioxide directly into the tank's water column, making it available for uptake by aquatic plants. Due to historical reasons, CO2 injection equipment was deemed "high tech" equipment and often the most expensive/complicated component of a planted tank system. The term has little to do with actual technology used. Despite the misnomer, "high tech" serves as a good reference of differentiation as CO2 injection has huge impact and implications for a planted tank.
Going 'high tech' generally implies an interest to grow more types of, and more demanding plants, and to grow them in a more optimal way to produce colours and density greater than what is possible without CO2 injection.
The CO2 system consists of a cylinder with a gas regulator attached, which injects Carbon dioxide into the tank's water. It is not as complicated as it sounds.
You should consider going "high tech" if:
How do you tell immediately that these are pictures from high tech tanks? Intensity of coloration from red plants and the density achieved here is an unmistakable sign of CO2 injection at work. These results are not achievable in a non-CO2 injected planted tank. There are a few red species that can grow well in low tech planted tank setup, but using CO2 gives much more options as well as better coloration. If growing red plants is one of your main goals, learning how to tune CO2 well is critical. A guide to growing red plants is covered here.
Many of the smaller carpeting plants are more demanding and require CO2 injection to spread and grow well. This includes species such as Hemianthus callitriodes 'cuba' (HC/dwarf baby tears), Glossostigma elatinoides, Eleocharis acicularis (Dwarf hair grass).
Under good conditions, carpet plants spread quickly and fill in most tanks in a couple of months. Many of these carpeting plants are small and more vulnerable to algae; having CO2 injection goes a long way into maintaining their health and resistance against algae. With good CO2 levels, less light is necessary for carpets to spread, as the plant can divert energy from CO2 capturing enzymes to vegetative growth instead. Given that more highly powered lighting systems are common on planted aquariums nowadays, low/inadequate CO2 is the most common reason why some aquarists fail in their carpet growing endeavours.
To find out how to grow HC, here is the plant guide.
Elongated internodes are common when plants are under stress of low CO2 levels; the plant channels more energy into vertical growth in order to breach the waterline and get access to gaseous CO2. When CO2 is adequately provided, aquatic plants take on denser growth forms, which is more aesthetic. Leaves and stems are also more robust, and less prone to algae.
In summary, go for injected CO2 if you want to grow more demanding plant types, including most carpets and coloured plants. CO2 injection also allows a larger variety of plants to be grown in high density. Because of improved plant health, algae issues are lessened/more easily managed.
CO2 injection really isn't that complicated to start - equipment has become cheaper, more standardised over the years and the science of how to manage it well has improved as well. The main downside is higher upfront equipment cost.
Additional budget for:
The startup cost of a standard injected CO2 system (CO2 tank + regulator + diffuser) is around US$250+. (In Asia, probably much less) For a 40gallon aquarium tank, a 5lb cylinder lasts for around 3-4 months at a good injection rate. Cost of CO2 refill for a 40 gallon tanks should cost less than $5 USD a month.
Establishing a regular nutrient dosing schedule to meet the high growth requirements is a necessity for CO2 injected tanks. This means a budget for fertilizers and a routine of dosing.
Regular trimming due to faster growth of plants. People underestimate this until it hits them in the face. Many aquatic stem plants WILL grow a few inches a week. We highly recommend planning ahead; using less stem plants and more slow growing species instead.
Greater attention to water changes & cleanliness to prevent algae, especially when using high powered lighting systems.
CO2 injection itself is a delicate art-form. There are many aspects to ensure CO2 injection is well-tuned. Compared to nutrient dosing, CO2 injection is trickier to manage as there is a much wider margin of error.
The in-depth section contains the various intricacies involved; managing gaseous exchange, gauging CO2 levels, methods of injection.