Will having CO2/ferts expand your effective light range.
Think of light as a gas pedal of a car. The harder you press down on the gas pedal (the more light you have), the faster your car will go (the faster your plants will grow). However, the car will require more regular maintenance (your plants will require more maintenance, i.e. fertilizers, CO2, etc).
Even if you dose fertilizers and inject CO2, you will not be able to grow a high light demanding plant in low light conditions.
If there are no fish (there are a few snails) do I have to worry about CO2 levels or can I just throw a DIY system on there and not worry about oxygen levels.
There is a common misconception that injecting CO2 will force out oxygen. This is not the case. You can inject much CO2 while the oxygen levels remain fairly stable. The reason fish/invertebrates die is because the CO2 can bind to hemoglobin, causing gas exchange to not occur (even though the water could have high levels of oxygen).
If you do not have any living organisms in your aquarium, you could crank your CO2 up as high as you want.
Is there such a thing as over fertilizing. In a tank with or without fish ( I also have a 55 planted with fish, moderate light but no CO2) is it possible to overdose the fertilization to the point where it negatively effects plants (in a non fish tank) or fish in an populated tank.
In a tank with fish, the more fertilizers you add, the higher the total dissolved solids (TDS) becomes. This can become stressful for fish.
In a planted tank without fish, excessive levels of nutrients would just be a waste. There is really no need to continue dosing if the plants aren't going to use it up (i.e. if your car gasoline tank is full, you do not keep putting gasoline into the car).
Will regular dosing of ferts and presumably healthy plants ever be enough to get rid of the BBA or once I have it even if it isnt spreading it will not go away on its own. Will the addition of pressurized CO2 cause the algae to die off? And if I add CO2 should I discontinue Excel, but continue on with the other ferts?
BBA is a difficult algae to treat once it has taken ahold. The presence of healthy plants will definitely discourage its growth, but you will likely need to manually remove the BBA to remove it. Healthy plants will then keep it from coming back.
You can always spot treat with Excel (using a syringe) to blast the BBA.
If you add CO2, you can discontinue Excel, as they are redundant. However, as mentioned, you can use Excel to spot treat to eliminate BBA. Finally, you will need to continue to add fertilizers, however.
If I ran CO2 would I be a fool to continue running the airpump, at least when the light was on? Or if it is pressurized CO2 put through a reactor, will it not matter.
I would turn the air pump off during the day if you are injeccting CO2. If you want ease of mind, you can always put the air pump on a timer so that it comes on at night.
Regardless if you use a reactor or not to diffuse the CO2, agitation of the surface of the water (i.e. via the air bubbles) will cause the CO2 to diffuse more quickly into the atmosphere.
The air pump is a model for a 20 gallon tank, so it isnt pumping out huge amounts of air, but I had shut them off when I first started using Excel, and I guess I had a ph crash (if thats possible with Excel)
It is not to my knowledge that Excel can cause the pH to crash.
So I am nervous about the whole CO2 / lower oxygen level issue...
As I mentioned, the injection of CO2 does not displace oxygen. A drop checker (with a 4 dkH reference solution) will be a useful tool in order to determine whether you are over injecting your CO2, however.