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1. Why do I need to inject CO2? Isn't my air diffuser/liquid supplement enough?
So I have read almost everywhere that CO2 injections are a must in med-high light tanks. I understand that plants need co2 to grow, but shouldn't the co2 from a balling air diffuser or liquid supplements be enough? If not, why?

You don't have to inject CO2 to have a successful planted tank, but CO2 makes everything "easier" and plants grow healthier and faster. CO2 also allows for stronger lighting creating more plant species options, better plant colour and reduces algae growth.

2. Why can high light and no co2 injection cause algae blooms?

Healthy plant growth suppresses algae. Thriving plants won't let algae grow on them, they will use up excess nutrient build up and uptake non-excess nutrients essentially out competing algae. The higher the light, the faster you are pushing plant growth. If CO2 is the limiting factor, the plants will struggle. Struggling plants create a zone in which algae can thrive. Algae can grow faster / better than plants when CO2 is limited.

3. Why does CO2 have an effect on ph and kh, and why should I be concerned?

Dissolves CO2 creates carbonic acid. Acids lower pH, thus when you inject CO2 you lower the pH. CO2 has no effect on KH.

4. Are all in one fertilizers enough, or should I be more meticulous about nutrients?
I have seen a lot about fertilizers and nutrients, but I am wondering how particular I need to be about giving my plants the right amounts. Would something like Sachem Flourish suffice?

All-in-one ferts are enough, however the longer you are in to planted tanks, the better you will understand plant needs and you'll venture into DIY ferts to better suit your tank's needs. No 2 tanks are the same. Seachem Flourish will work if 1) you use their entire range (Nitrogen, Potassium, Phosphate, Comprehensive and Trace) and 2) you have enough money to keep up with the amounts of Flourish you'll go through. Generally, for the same money, you can get a years worth + of DIY fertilizers to a month's worth of Flourish.

5. Will potting soil work as a substrate?
I have seen many posts about using potting soil, such as miracle grow organic, as a substrate. As long as I perp it properly (by adding and draining water and removing floating debris), will it provide the proper nutrients?

Potting soil works yes, however you are placing a lot of incontrollable amounts of organic waste under the gravel / sand cap. The nutrients will release into the water column and will be used up and eventually you'll have to tear the tank down or fertilize anyway. Algae thrives in organic waste rich tanks. To keep things "clean" a lot of successful tanks use inert substrates, and then fertilize with quality fertilizers. They keep the substrate clean thus limiting organic waste from accumulating in the tank.

6. What kind of light should I go for?
Amazon has hundreds of aquarium lights with at least 4-star reviews. These lights can range from 20-200$. What should I look for in lights for planted tanks? Any suggestions on lights for a 24in deep tank?

1) Do some research into LED's vs T5-HO. Those are the most common lights that are used today. Many us us (myself included) use T5-HO, and others use LEDs.
2) Find some PAR numbers for the light you are looking at. PAR is the strength of light. 30PAR or less is low light, 30-60PAR is medium light and 60PAR or greater is high light.
3) Colour spectrums are important. Finding a nice looking light while maintaining a proper colour spectrum for the plants is key. That's why many of us use multiple T5-HO bulbs. Some bulbs are for PAR and the plants, and other bulbs are for making to entire tank look pleasing to the eye. Other bulbs are for both. LED's can do the same thing as long as they are controllable. Generally, the higher the price, the more controllable an LED light will be.

Sorry for the long post, I just have struggled to find a comprehensive guide that wasn't too basic or completely over my head. My last attempt at growing plants was a complete failure; even the hardiest plants never lasted more than a month. This time I want to do it right.
Thank you guys so much and Happy Thanksgiving!

See bold ^^^

Dennis's website is a very very good reference for most things related to planted tanks. I agree with everyone above you should spend some time reading his articles.
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