Hello Ken:
FWIW....I do not believe the roundup (glyphosate) was directly responsible for the fish kill you mention above.
In regard to the need for aeration, I highly recommend either purchasing or finding someone with a good dissolved oxygen meter. I personally use a YSI ProODO meter for monitoring dissolved oxygen and temperatures in lakes. There is no mention of DO concentrations in your posts so no one really knows what if any level of aeration is needed in your situation. Bottom sediment composition, run off from surrounding watershed, fertility, etc, all play an important role in determining the need for aeration.
While whole lake aeration can be beneficial in certain situations, it can also promote continuous planktonic algal blooms. Lakes with high sedimentary phosphorus concentrations are particularly prone to these types of aeration induced blooms. The concept of aeration reducing algal blooms is based on the reduction of phosphorus as it binds to ferrous iron in the water column.....where it precipitates out as ferric phosphate. If iron is limiting......the chemical reaction obviously does not take place. When iron becomes limiting.....large amounts of sedimentary phosphorus can begin cycling in throughout the water column....making it available to undesirable algae.....thereby increasing the BOD (biological oxygen demand) of the lake, pond, etc..
In summary.....obtain dissolved oxygen data for your lake. If I were you....I would purchase my own meter considering there are times that some of your fish kills were related to agricultural runoff.....(you can then monitor the specific effects of the runoff at any given time) By having your own meter, you can design an aeration system (if needed) to your particular needs. This will save you in both product and electrical costs.
Regards,
Jeff Gray
Grays Aquatic Services
www.GraysAquatic.com