Diffusing CO2 into the tank generally uses one of the following types of equipment:
The most conventional device; it can be effective if used properly. The finer the mist the better. Additional item in tank distracts from tank aesthetics. Clogs easily and require regular servicing to be stable.
Good ones produce a smoke-like fine mist. Requires far less frequent cleaning than in tank devices. Easy to tune and does not add additional stress on the filter. Mist can be visually distracting.
Very efficient compared to atomizers/diffusers and for people that can't stand CO2 mist. But can be hard to calibrate. Especially for larger tanks. Needs a good filter flow rate to operate well if used on the outflow line.
The most conventional device, it can be effective if used properly. Ideally, you should aim to achieve a fine stream of mist. It requires alot more servicing than inline devices due to exposure to the tank environment (i.e will get clogged due to detritus etc.) It has more weaknesses compared to other methods. If you are using am in-tank diffuser, these pointers may help:
Placement is critical
Placement in the tank is important. Diffuser should be placed in the down wash current; see picture below. Given the circulation flow pattern produced by the filter outflow; diffuser should be placed at where the blue X is.
The current should carry the bubbles from the blue all the way to the other end of the tank. If bubbles rise vertically from X instead of floating down with the down current, it means that either the diffuser is ineffective; producing bubbles that are too large, or that the amount of flow in the tank is too weak.
The downside of in-tank diffusers is that their efficacy relies on their cleanliness, as well as consistency of flow pattern in the tank. As the diffuser clogs, efficiency is affected. Similarly, filters can slow down as they get clogged, affecting flow. Due to most hobbyists being not super-consistent with tank cleanliness, this often results in fluctuating CO2 levels. Being in tank also means that it can be blocked by plant growth.
As mentioned, efficiency rate of CO2 diffuser in tank is affected by changes in flow pattern. For example, tanks using HOB filters where flow pattern change with the water level, or tanks where dense plant growth change the flow patterns over time will face unstable CO2 levels.
Even large sized tanks can be run using in-tank diffusers (if they are serviced regularly and flow is not obstructed).
Good ones produce a smoke-like fine mist.
Atomizers require far less frequent cleaning than in tank devices, and are less affected by the flow pattern changes in the tank unlike in-tank devices. We find no reason to have more equipment in the tank; thus we will always prefer using inline atomizers compared to in-tank devices. The main downside of such devices is that the mist can be visually distracting - many people prefer clear tanks. The advantage of having constant mist is that one can observe the flow patterns in the tank; whether flow hits all plants adequately.
Atomizers vary in quality; some produce a very fine smoke-like mist, while the ones that are not so well made produce more visible bubbles. We have not seen a brand that is 100% consistent, when buying atomizers we advise buying from a shop that allows returns should the atomizer produce a mist that is not as fine as desired.
Unlike reactors that work within certain flow & injection rates, inline atomizers can handle huge amounts of CO2 input. A single (well designed) atomizer is enough to fully saturate a 1000 gallon (3800L~) tank at normal injection rates provided it is channeled through a strong current.
In a similar manner as diffusers, atomizers can be cleaned by soaking in bleach or H2O2 solution. Because they are not within the tank environment, they require much less servicing - once every 2 - 3 months. This is one of the main advantages of Inline devices. Some inline atomizers can be more easily dismantled depending on their design.
Inline-atomizers and inline reactors reduce the need for equipment in crowded tanks. (sometimes equipment is even more obvious in small tanks) we use inline atomizers in most of our setups. In the 9.5 gallon (36L) tank above, both the surface skimmer and glass outflow pipe are hidden behind the hardscape, and an inline atomizer is used to inject CO2 into the tank. Without the need to remove any equipment, the tank has a very clean look.
Reactors aim for a 100% dissolution rate.
They are very efficient compared to atomizers/diffusers and for people that can't stand CO2 mist in the tank, they are the ideal choice. Their efficiency and dissolution rate depends on the design of the particular reactor, and the flow rate of the filter/pump that powers it. You can use it attached to filter outputs, but they do reduce flow slightly.
Reactors can only handle a certain maximum injection rate for their design/size - so getting the correct size for your tank matters. If your tank is larger than 250 litres, most commercial reactors shorter than the length of your arm will not be optimized to dissolve CO2 at a high injection into your tank. You would need to look for specialist merchants or DIY reactors that are larger in size, paired with a way to get enough current through them. Many folks that buy small sized commercial reactors for their large tanks end up unable to hit good saturation rates.
DIY reactors (rex grigg etc) can also be made cheaply with PVC piping. They are not much more expensive compared atomizers, despite the large apparent bulk.
The only downside of using reactors is that they do not produce CO2 mist, which plants can utilize directly. Many tanks are grown well without mist application, so it's not a big worry. Reactors should be the default choice for tanks if you do not want mist.
Advantage of CO2 Mist
The main downside of misting is that it can be unsightly to have bubbles obstruct the view of the tank.
The upside is that plants can use CO2 mist directly for photosynthesis according to data in controlled experiments published by Barr report. This allows us to give plants CO2 above and beyond the max saturation rates of dissolved carbon dioxide that may impact livestock respiration - this is the main advantage of having CO2 mist. Barr report reported an increment of 25% Oxygen production (used as a proxy for photosynthesis) in sample that had access to CO2 mist - compared to sample that did not; even as the two test samples had equal levels of dissolved CO2 in the water column.
Misting is not a necessity in most tanks but we find it gives an edge with difficult species or plants weakened by poor transport etc. For most tanks, reaching a good rate of dissolved CO2 is sufficient to grow most species.
Another angle to view CO2 mist is that it allows one to have a lower rate of dissolved CO2 while still getting carbon dioxide to the plants. This special scenario may apply when one keeps very sensitive livestock that prefer a lower dissolved CO2 level in the tank.
The importance of flow
Especially for mist users, flow patterns in the aquarium greatly impact the efficacy of CO2 delivery. Some clues that you need to adjust / improve flow:
Some areas plants grow very well, while some areas have very poor plant growth, despite getting similar amount of light & fertilization. This is more obvious with difficult plants - which usually do visibly better in open areas with good flow, and do less well along the aquarium sides.
The CO2 mist is confined to a certain area, or you observe that the mist from diffuser/atomizers rises vertically from diffuser.
Diffusers should always be placed in the down wash of the water outflow current; bubbles should be fine enough to get pulled by the current downwards when they first exit the diffuser.
There should be good distribution of mist- else it may hint that the diffuser/atomizer doesn't produce fine enough mist, or that the current is too weak.
Algae in specific areas
In many setups, specific algae types exist in specific areas of the tank.
For BBA, it can indicate areas of too turbulent flow (with CO2 mist). Having outlets that give a more even distribution may help.
Cladophora and hair algae can accumulate in areas where detritus build up due to flow pattern. Planting more robust plants in problem areas, and doing more cleaning to clear detritus build up can help. Planting more robust plants in poor flow areas - where demanding plants don't grow as well, is one way to by-pass flow issues.
Using Hang-On-Back Filters
HOB outlet should flow horizontal rather than down to travel across length of tank. The down wash current should be strong enough to pressure the CO2 bubbles against the substrate, and allow the mist to travel the entire length of the tank back to the filter inflow area.
This will not work if (a) the diffuser does not produce fine enough bubbles, (b) the flow is not strong enough, or (c) the flow pattern is wrong (HOB flow points downwards instead of horizontal).
Same situation applies if using canister outflows instead of HOB.
Read here for more about algae control for BBA and cladophora etc.