there is no need to ask or to fret, just run that ad or a similar one every 2 months, everybody will understand and those who don't should get banned anyway. I don't think you can avoid this at your rate of growth, so just embrace it sooner to avoid too many of these 'events'.
P.S. when the site was running badly i would often get errors when going to the next chapter, it would tell me to wait and reload, which is what i did, but it reloaded the chapter i already read, not the next one.
I don't mind, but it's just wasting the servers resources, multiply that by 20.000 and that results in a lot of strain that could be reduced.