Her first word of advice is not to stock tanks too quickly - wait for them to cycle. When I first kept fish, back around 1980, I was unaware of the concept. I don't recall it being in any of my fish books. When I returned to the hobby last year, it was something I was made aware of almost immediately - quite rightly, it's one of the first things they tell you are the pet store. That said, I still messed up with the whole cycling thing - it would have saved me (and my platies) a lot of stress if I had understood the time course a little better. I had fin rot and "shimmying" fish - because the tank wasn't yet cycled. While everyone wants fish as soon as possible, fishless cycling is much less stressful on the aquarist. But who really knows that right when they start off?
Once you have a tank that has cycled, you still need to add new fish slowly. More fish means a larger input of nitrogenous compounds. Since the bacterial populations will be limited by food availability, adding too many fish too quickly, even to an established tank, can cause a spike in ammonia or nitrite levels.
Her next piece of advice seems pretty straightforward as well - don't overfeed. But overfeeding is an elusive idea. If you read a pack of fish food they will say "no more than the fish can consume in three minutes". But that really depends on the fish. Some fish will take a couple minutes to even notice the food. Others will just keep eating. If I could get flakes that would float for three minutes, I suspect that my Macropodus would consume their own body weight in food.
Youngs has some interesting advice about feeding that I hadn't come across before. "How often should I feed my fish?" is a common question. Some people say three times a day, some people say once a day or less. Why such diversity of answers? Probably because it depends on the fish you're keeping. Youngs advises feeding small fish like tetras and guppies several times through the day. Larger fish should be fed less frequently. In addition, herbivores need to eat more often than carnivores. This is all pretty obvious once you think it through, and it's probably something a lot of people know intuitively. But again, it isn't something I remember coming across before.
She has some fairly standard things to say about lighting and algae, but again, says them well. Good advice on acclimating your fish, and on suitable tank mates. She advises redundancy when it comes to heating and filtration - two filters, two small heaters. Why small heaters? Because if one sticks on the "on" position, it won't heat the tank up too quickly. Common sense. Hadn't thought of it.
The last point she makes has to do with stocking levels. If you visit discussion boards, most people talk about the 1 inch per gallon rule as the upper level when it comes to stocking, which is why I was surprised when I read that an article in the February issue of Tropical Fish Hobbyist which suggested that you start a 10-gallon tank with "10 neon tetra-sized fish" (1 inch per gallon) but that you could eventually go to double that once your tank was well-established. Bending the rule a little - I'm sure most people do that. But going to double that? I was surprised. So it was nice to see Karen Youngs give similar advice
Nowadays with better filtration it is hard to give an exact figure, but PFK recommend the following:Getting that sort of advice from one source seemed iffy to me. Getting it from a second, independent source (and getting a more nuanced answer) makes me feel a little more inclined to integrate that factoid to my pool of knowledge.
Tropicals: 1” per gal/ 2.5 cm per 4.55 l initially, then up to a maximum of 2” per gal/5 cm per 4.55 l after six months.
Coldwater: 1” per gal/2.5 cm per 4.55 l.
Marines (fish and inverts):
1” per 4 gal/2.5 cm per 18 l.
Marines (fish only): 1” per
2 gal/2.5 cm per 9 l.
Ponds: 10” per 100 gal/25 cm per 455 l.