A lot of things 'worked' in the old days, but that doesn't mean that what they did was 'right'. For example, in days gone by folks would 'preserve' many low acid foods like green beans in ways that we would consider dangerous today. Many people did it and were just fine, however, there were also many people who did it who got sick and/or died from it. Yet, some folks point to their grandparents who did it like that and never had a problem as 'proof' that the old way worked/works. However, for every one that didn't have a problem, there so many that did.
Going back a bit further, doctors would use 'bleeding' as means to 'cure' patients who suffered from fevers. And some of the patients who were treated this way survived, but many others succumbed to the illness, as well. It took a great many years to change the practice, because the treatment 'was successful' for some patients, even though there was no scientific explanation for how the treatment in any way benefited the patient and was actually detrimental to their recovery.
Fish, back when your dad was keeping them, were viewed (and to some extent still viewed) as 'expendable' pets. I've had many conversations with folks who think that keeping a fish alive for 3-6 months is a great accomplishment. Or that if they were successful in getting livebearers to have fry successfully grow to maturity that they have somehow achieved something. The truth of the matter is that most fish in the hobby should live a good number of YEARS if kept properly. Some, live 2 years, some 7 years, others up to 20+ years.
Ultimately, the point is, scientific understanding of the aquatic biosphere has advanced and techniques for fishkeeping have changed to align closer to the understanding.