I have been using Flourish Excel in all my planted tanks, except in the one I used to have with pressurized CO2 added. While my experience is anecdotal. I have not only not seen negative effects on most of the plants, I have had fish in some of these tanks for 20 years. So I will say that I disagree with people who post that this product is bad or dangerous for most plants or fish. But, there are a few exce[tions and over dosing this product will increase this potential for such harm.
On the other hand I respect the right of folks to decide not to use it, but i also would love to see any scientific proof that Excel is harmful when used according the the directions. I do not consider it an algaecide. What I do know is that the algae is not able to utilize it the way the plants can. So, this helps he plants to out compete algae in many cases.
It is true that Excel, when applied directly to BBA, kills it but other forms of algae do not suffer the sane result. However, healthy growing plants are often the best method for the prevention of algae and Excel can help with this side of the algae reduction equation. But, I would also note that the application of Peroxide in a similar fashion will also kill BBA.
However, there are some plants and even some species of fish that do not do well with Excel, especially ifn it is over dosed. I never had a problem as I only add Excel after a water change and then at the rate of 3 ml/10 gallons of water. This is less than the dosage suggested by Seachem.
I have 1, 3 and 5 ml disposable pipets, so I can measure my dose pretty accurately and also use them for feretilizer levels.
If one has decent surface agitation you should have close to the maximum amount of dissolved CO2 in your water unless you have heavy planting with lots of plants which use up the CO2 faster than surface agitation can replace it. However, there are other factors which can affect the CO2 level in water. For one our fish exhale CO2. Organics decomposing can also create CO2. And then pH levels and temperature can also have an effect on CO2 levels.
Depending on your budget there are different ways to measure CO2 levels in water. The more accuracy one needs, the greater the cost of measuring becomes. However, the use of a liquid CO2 substitute will not register when one tried to measure CO2 levels in water. However, the most common way to do so is by using a drop checker.
If you want more information on checking CO2 levels, here s a good read
https://www.2hraquarist.com/blogs/choosing-co2-why/co2-fine-tuning-3-techniques
One last comment. the level of CO2 in one's tap water can vary. Water from deep underground can contain insanely high levels of CO2. When I set up my first tank I was pretty ignorant about testing etc. I measured my tap pH in the low 6s. And I thought that was the true number. I did not check my tank water for a while. When I did I discovered my tank pH was actually more like 7.4. I have well water and it turned out that, when it came into the house from underground, it contained a high level of CO2 which will cause pH to be lowered,
Once the excess CO2 was out-gassed by agitating the water as it came from the tap, went into buckets this process began. It was completed when the filtration agitated the water surface in the tank which allowed any remaining excess CO2 to out-gas and the result was a rise in the pH to its "real" levels. This is why you often see the instructions for measuring you tap pH telling you to agitate the water sample before testing. Alternatively you can just let the sample water sit for a bunch of time to allow any excess CO2 to leave the water. I use an airstone bubbling in it for about 5 minutes to achieve the needed result. Th bigger the sample the more agitation time that may be needed.
Finally, one of the best signs for determining that one's CO2 levels are too low is plant growth will slow or even stop. We may even see some disolorong of the plant leaves as well.
