Frogface, Thank you for the kind words about my article.
The answer to your question depends on several factors.
The first factor has to do with the life cycle of the slugs.
Slugs / Snails have an interesting lifecycle.
There are three stages in the life cycle of a slug / snail: the egg, the immature stage and the adult. An interesting point of note is that each life stage of these critters can survive through the cold winter without a problem.
Now most Slugs and Snails are hermaphrodites having both male and female organs, so every individual can lay eggs and they can even self-fertilize. With this type of a system, you can raise a lot of slugs in a short period of time.
Garden Snails fertilizing each other.
These critters are so prolific that one individual has the potential to produce 40,000 or offspring throughout it's life, having up to 300 clutches, with 10 to 50 eggs per clutch. They prefer to deposit their eggs in moist, but not waterlogged areas. This is why they do so well in our tanks. They drop their eggs into any crack or crevice can find, hiding them under stones, sods of grass, attaching them to leaves, etc., any protected but moist place that they can find. We typically will only see 1 out of every 10 slugs that are in our tank. This is why the lettuce method of removing slugs tends to be so ineffective.
Their eggs look like little white / tan pearls, being gelatinous, and watery, usually spherical in shape, measuring about 3 - 4mm across.
The development period of the eggs can vary greatly depending on the temperature and humidity. During warmer humid weather they can hatch as quickly as 10 days, just what they find in our little tropical islands of paradise. But when cooler conditions prevail maturation can take up to 100 days or more to complete. Also, eggs that are laid in the fall will typically survive the winter to hatch the following spring, just as soon as the weather warms back up.
During this stage of their life, a slug egg is much more resistant to hypoxia and suffocation. Therefore the CO2 bombing may not, likely will not kill all of the eggs. The higher levels of CO2 will slow down their development, but it seems that it will not kill all of the eggs without prolonged exposure to high levels of CO2. If you have plants in your tank this will be difficult to achieve.
A Nuscince is Born
After hatching the slug typically matures in less than a year, with most slugs taking about 5 months or so to reach adulthood. But if conditions aren't right it can take them up to 2 years to reach maturity. After puberty, they can live for two or more years as adults.
After each bombing, I leave the CO2 in the tanks over night with the glass lid left covering and closing off the tank. This will keep air currents and drafts from washing / blowing the CO2 out of the tank. Also plants love the high CO2 levels. In the presence of light, they will metabolize all of the CO2 they can get their leaves on. Therefore I turn the tank's lights off so that the plants photosynthesize less and which pulls out a lot less of the CO2 then if I leave the lights on.
Also a few folks who have experimented with CO2 have reported that only 8 out of 10 slugs and snails have actually been killed after a 24 hour exposure period to CO2. Therefore two out of ten slugs just might wake up from the CO2 kiss of death.
All of these factors go into why I choose to bomb my tanks a second time the next day. That wayI figure I will kill any remaining adolescents and adult slugs.
I found that even with bombing two days in a row, my tanks still had a few springtails that were likely to survive the bombing as I always have found a few springtails crawling around the tank the next day after the second bombing.
Now to kill the Hatchlings
I then wait a couple of weeks for any eggs to hatch out and then bomb the tank once more. This seemed to kill all of the slugs and slugs and snails, and the plants loved it too.
Reseed the Tank
Lastly I would reseed the tank with isopods and springtails, then put the frogs back into the tank.