What's the most inexpensive way to get co2 into the tank for plants?
I've been told that liquid co2 and diffused co2 do different jobs, is that accurate?
TLDR:
Pressurised CO2 for long term dense plant growth under high light.
Liquid CO2, useful in a pinch and for fighting algae outbreaks but other than that not recommended.
If you are genuinely running out of CO2, dim your lights until you can afford a proper pressurised system.
Sort of, both 'liquid CO2' and diffused CO2 do both increase the available carbon to plants which is what you want. But you really need to ask yourself if you actually need more carbon. There will be carbon naturally in the water from the natural gas exchange with the air. You'll only run out if you have a lot of plants or have lots of light. The chances are if you are running 'stock' lighting on your tank you probably don't need it.
Liquid CO2 is cheaper in the sort term but is more expensive in the long term due to the upfront cost of getting a decent regulator and putting a deposit down on a CO2 bottle. You can get systems that use disposable cartridges but honestly, I wouldn't bother. The system isn't much cheaper than buying a proper regulator, the re-fills are expensive and don't last long at all.
Upsides of a pressurised system is that it is set and forget, it takes a little fine tuning at the start but once it's done you only need to touch it to change the tank over. You run the CO2 on a timer which you set to come on 1-3 hours before the lights and shut off just before the lights do. The length of time your tank lasts obviously depends on the size of it, the size of your tank and how much CO2 your pushing. A 3KG tank on my 180 litre lasts 9+ months and costs about £15 to refill. Downside is the initial cost. The whole system was about £200 to set up all in, £50 for bottle deposit, £15 initial fill and about £130 for the regulator, piping, drop checker, diffuse etc. If your running high light this is the only method you should be considering IMO, its the only way to get consistently high levels of CO2 for optimum plant growth. The larger the tank, the quicker the payback period.
Compare that to 'liquid CO2', the upside is that it has no upfront cost but it is expensive to run. You have to dose it daily for it to be effective, that means using a dosing pump if your forgetful. Another plus is that it is pretty effective at killing algae in higher concentrations, you can 'spot dose' it during water changes on stuff like anubias which tends to get algae. Some plants don't like it and do melt/die even at low concentrations. Some fish also don't like it but I have personally never had any problems but your mileage may vary. Don't quote me on this but I seem to remember reading it has a very short life once its in the water so it really needs to be going in before the lights come on to be effective.
The main issue that I have is that I don't really find it effective at the recommended dosage, it just doesn't add enough carbon for a heavily planted tank. It's inconsistent because its 'dosed' and often forgotten. People do use it in higher concentrations (2X or even more) but I really don't recommend it, the costs rack up quickly and there is a risk of fish loss and plant loss. The active ingredient is glutaraldehyde, this is actually an industrial cleaner and disinfectant hence the 'do not overdose' warning. It is possible to kill a tank if you or someone else makes a mistake when dosing (like if you went on holiday...). I just keep a bottle to use in a pinch for fighting any algae outbreaks but other than that I don't use it.
Ill 3d print a small bracket to take the extra weight and remove the cellotape. (Or maybe the cellotape will stay for this year who knows)