I just got back from my LFS which I find to be very knowledgeable. In
disagreement with some theories I have read here, the LFS said that CO2
would make an algae problem worse, not help it. And he supported his
thought.

How? He may be well versed in some areas, but he's not here.

Now, I what like to know what is going on with CO2, algae and
photosynthesis. What kind of results should I expect?

Well being someone is well versed in many things algal, all algae can
and do use HCO3, or bicarbonate for their source of Carbon. A small
number of plants can do this as well, Hydrillia, Egeria, Potamogeton,
Vals but they grow much better with CO2 added. All plants do. Most
algae do not. A few such as Green water, A couple of species of
Cladophora, Chara do better with CO2. Most do not though.
What happens when CO2 is added to 15+ppm they start photorespiration.
They have no protection from photorespiration radicals (Oxygen
radicals that are extremely destructive).

Plants and Chara possess perixsomes, these remove these radicals and
improves efficiency. Other algae do not have these oraganelles.

Plants have evolved using CO2 and algae have stuck with HCO3. HCO3 is
your KH.

Plants use and need far more Carbon than algae. Even the worst algae
bloom produces very little biomass vs a well growing plant tank.

Since most plants can only use CO2 vs HCO3, adding it allows the
plants to grow. Plants are roughly 40% carbon. Limiting this nutrient
will slow plant growth down but the algae have no problem at all
growing.

So you shopuld except things like great plant growth, "pearling",
reduced algae issues etc if you use it properly and raise the
dissolved CO2 levels to 20-30ppm.

One thing that many do not realize, adding CO2 gas from gas tanks, DIY
yeast ect, DOES NOT DISPLACE Oxygen. Many think they are dependant on
each other. In planted tanks, O2 is above 100% most of the time, so is
the CO2(all of the time). Fish/bacteria like the O2. Too much CO2 can
cause some problems but 30ppm to 20ppm works very well and has no
effect on fish, inverts, bacteria etc.

Regards,
Tom Barr