Good reasons why you don't need pressurised CO2 injection to have success with aquatic plants. This will also act as a list of plants to try if you are not having much success...
I thought you might all be interested to hear another of my (many) good reasons why you don't need pressurised CO2 injection to have success with aquatic plants. This will also act as a list of plants to try if you are not having much success...
Let's start by getting a few myths busted...
Aquarium plants are grown in the nurseries emerged (out of water). This is done so that the plants grow rapidly and more robustly (no water to help hold them up) which means they transport better.
Most 'aquarium plants' do not spend most of their lives underwater in the wild. Some grow on rocks in the moist environments at the side of waterfalls like buces (left image) and others at the side of rivers where they occasionally get flooded in the rainy season like crypts (right image).
If most aquarium plants grow faster and stronger out of water it will be no surprise then that most aquarium plants are not readily adapted to underwater growth. Plants have evolved over time to work as efficiently as possible with access to atmospheric CO2. Whilst CO2 dissolves quite readily in water and most plants struggle to survive underwater (submerged). Some plants will adjust better to underwater growth like those crypts. They are famous for melting as soon as you place them in your aquarium. Do not despair though, this is just them replacing their emerged leaves with new submerged leaves more readily adapted to underwater growth. The images below of the Rotala Rotundifolia show this process well. By comparing the emerged (right image) and submerged (left image) growth forms we can see how differently the plant has 'set itself up' to suit the environmental conditions it is facing.
Further to this though, some plants have adapted to underwater growth permanently and can no longer grow out of water. These are the true aquatic plants I refer to in the title of this piece and are naturally some of the easiest plants to grow in our aquariums. Now bear with me for a moment whilst I do the science bit... these plant can use CO2 from the water but can also make use of bicarbonates (commonly HCO3). They convert this to CO2 leaving behind HO ions (these are a catalyst site and readily react to form calcium compounds which can in very high light conditions leave a sandy substance on their leaves (this is known as biogenic decalcification). This allows them to take advantage of these more commonly available compounds underwater for them to perform photosynthesis. These plants often occur in hard water areas where there are more bicarbonates present - dissolved limestone for example is Ca(HCO3)2 and so will naturally do better if you have hard water too.
So what plants should you look out for if you want to try some of these truly aquatic plants. By the way this isn't so say that you will struggle with other not-truly aquatic plants in a low energy system without pressurised CO2, all the plants mentioned so far are likely to work great too (Rotala is a bit trickier in my experience) as will others in growers "easy" ranges.
That's not to say these plants cannot benefit from additional CO2 because they still have the ability to undertake 'regular' photosynthesis as the above image shows which is of a piece or Hornwort which I have momentarily exposed to atmospheric CO2 by removing it from my aquarium and replacing it. You can see the intense release of oxygen, otherwise known as pearling when the surrounding water become over saturated with oxygen and it cannot dissolve quickly eough and so gasses off. This effect is normally only seen in high energy aquascapes though even then, not normally to this level. You may have noticed, that these plants are often sold in the pond section as oxygenators. They are great in this regard too for aiding the respiration of your fish. They also show a preference towards ammonia rather than nitrite or nitrate as the usual plant food sources. If you know a little about the nitrogen cycle performed by your filter you will therefore know that these plants can help support a health tank and filter system additionally by reducing ammonia and nitrates in the aquarium.
Not only will these species be easier to grow, they are great at using up excess nutrients and outgrowing algae in the aquarium and so great at helping improve the overall look of your aquarium and reducing maintenance requirements. As such they can be used as 'supporting' or 'auxiliary' plants when starting a new aquarium, to reduce the likelihood of diatoms which plague new setups. They are fast growing and do need a regular trim, through they are pretty indestructible so you don't have to overthink this part of the process. As they often grow very tall, in an effort to make best use of the sunlight available in often tannin rich waters in the wild, they are best used as background plants. There are some images below of how I've used Vallis in the past below.
Thanks for reading, please do let me know if you have any questions, and I hope using these truly aquatic plants helps you create an 'easy aquascape'.