Selecting Plants and Fish for Low-Tech or DIY CO2 Systems

Guide to selecting and caring for fish and plants in low-tech systems.

If you’ve decided that a CO2-free system or DIY CO2 is right for you, it’s time to start planning your planting and livestock - the fun part! It’s still very possible to have a gorgeous planted tank with this setup with a little extra planning. This includes selecting plant species that are compatible with lower CO2 levels, aquascaping strategically and tailoring your plant care regimen to this setup. Check out our Knowledge Base for lighting and CO2 requirements for plants. This is one of the easiest ways to get an idea of what to buy.

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Fish are not as affected by a low-tech setup, but we’ve also included some notes on fish selection that will help you select the perfect livestock for your new tank.

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The science behind CO2
As we discuss in our low-tech setup guide, plants require CO2 as a carbon source to produce sugar using light energy. This process is known as photosynthesis. In nature, aquatic plants do not grow fully submerged, so acquire CO2 from the air, which has a much higher CO2 content than water.

For plants to grow quickly when fully submerged, it is necessary to supplement additional CO2. In most setups, CO2 is the rate-limiting factor for photosynthesis in home aquariums. It is possible to grow plants without additional CO2 or with lower-tech alternatives such as DIY CO2 or Flourish Excel, but when selecting and caring for plants, we have to keep in mind that plants will be photosynthesizing more slowly in these setups and plan accordingly.

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Selecting plants and aquascaping
With these factors in mind, let’s move forward to plant selection. In general, you will want to avoid plants that are considered to be high-light species. If a plant species requires high light levels to grow, you can deduce that it likely has a high rate of photosynthesis, meaning that it will not fare well when the rate-limiting factor of CO2 is not supplemented in abundance.

Plant species that thrive in low light levels, such as anubias and java ferns, are likely to flourish in a lower-tech setup, so are ideal midground species. Many foreground plants are high-light species, but mosses, such as java moss, are likely to do well in a low-tech setup. For background, we recommend fast-growing, low-maintenance stem plants such as pennywort, anacharis and potentially hornwort.

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If you’re just getting started the hobby, it’s tempting to start out with just a few plants to see how they do. We actually recommend against this approach, as starting out with a robustly planted tank makes nutrient imbalances and algae blooms are less likely and help establish a biofilter, meaning fish are likely to fare better. Ideally, when your tank is fully aquascaped, you should see about 25% of the substrate when looking down from above. Most of the plants at big name stores are meant for non-CO2 methods. If you're just starting out, you can start with these. Just be sure to wash the plants very very well to rid of any chemical treatment they may have received.

As we detail in our low-tech setup guide, plant care should also be tailored to accommodate a lower rate of photosynthesis. Lower light levels and a lighter fertilization regimen will help your plants to thrive in these setups.

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Selecting compatible fish species
Fish do not require CO2, and in fact are susceptible to overdoses from pressurized CO2 systems. You, therefore, have much more wiggle room in terms of fish species.

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In the case of a non-CO2 system, you will not be restricted at all in terms of fish selection. In this case, we recommend starting out slowly, adding hardier fish species first a few at a time. Algae-eating fish may also help prevent algae blooms in a newly established tank. If you are dosing Flourish Excel, keep in mind that livestock is susceptible to overdose, and stick with hardier species until you have established a dosing regimen for your tank.

If you are using a DIY CO2 system, it’s important to remember that the amount of CO2 production cannot be regulated by these systems. Therefore, an overdose is possible, especially in nano tanks or when using DIY CO2 setups such as sodium bicarbonate reactors. In this case, it may be best to stick with hardier fish species that are considered beginner species for a few months. We have noted that rasbora is particularly susceptible to CO2 overdose, so it may be best to wait to add this species in particular.

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