As far as the book you read, many people with good intentions pass on incorrect information, and it is very hard for new hobbyist to see the difference. I would suggest some deeper study into the lifestyle of the bacteria we are concerned with. the primary requirements of this bacteria are food (ammonia or nitrite respectively) oxygen, and a surface to cling to. Filters provide the best of all three and therefore contain the most bacteria in a concentrated space as a rule. Therefore filter cleaning should be approached carefully, one of the reasons many folks here always reccomend cleaning in tank water and nothing else. and also why complete media replacement is seldom reccomended.
Water is a poor surface for bacteria to cling to. bacteria may travel through the water but does not typicaly colonize and reproduce in the open water. therefore high volume frequent water changes ( assuming equal parramiters) will have virtually no effect on the in tank bacteria colony.
Gravel does provide a good surface, but unless ugf is used, the gravel is greatly limited by oxygen. so it generally stand as a secondary bio-filter with the primary tank filter being the biggest colony. In the case of UGF the substrate is the filter media so either way the filter is the primary.
When vaccuming the bacteria is not removed in large quantities. it will not shake loose that easily. bacteria lives in and clings to the tiniest of pores on any surface, and is very difficult to wash loose in relity. the only time bacteria is really reduced via vaccuming is when there are high volumes of mulm to the extent that they block o2 to the gravel, and the bacteria colonizes on the mulm being removed. in these cases, the tank really needs to be vaccumed and partialed until it is both clean and stable anyhow.
So improper cleaning of media is dangerous, water changes and vaccuming have little or no effect. therefore doing all three isn't much different than only cleaning your filters.
I am interested in the title of the book you read, and will most likely read it.
There is a very common and prevalent myth that bacteria lives in the water of the tank in significant quantities. This simply isn't true. there is also a common myth that vaccuming removes bacteria from the gravel this is also not true. It is not at all uncommon in this hobby for someone to write a book based on false information and incorrect deductions without scientific proof. I am far more concerned that new folks in this hobby would be scared away from good tank maintenance, we commonly see folks here with big problems that can easily be fixed with cleaning and partials, and many of them have been told repeatedly not to do too many water changes.
Lastly, I would look carefully at all possibilities that may have spiked the tank, given that the water changes and vaccuming couldn't have a large effect, and I imagine your filter cleaning practices are the same as they have been in the past ( with no problem reprted) there is quite probably a new variable thrown in somewhere.
Such as: Did you forget to dechlorinate (I've done it more than once in my life)
Did someone spray fabreeze in the house?
Did your water company change to chloramines?
Is it possible that anything else could have been introduced via your hands or tools or sink used.
there are many many possibilities to consider when these things occur.
There are many possibilities, and of course there is a definate cause but it may be hard to find. as soon as the fish died in the tank, ammonia would have been produced at an extremely accellerated rate, so ammonia testing after the fact is highly arbitrary. likewise if nitrite took a large spike, it is highly probable that you still had a good supply of bacteria. something had to process anough nitrite to cause a spike in less than 24hours. Typically even with fully stocked uncycled tanks, it takes two to three days to see significant ammonia levels. therefore to see the levels raise overnight enough to kill a tank full of fish seems unlikely to me. Consider all likely possibilities.
dave
