I honestly don't remember the mistakes I made as a newbie, like when I was 8 and had my first planted tanks. Nitrite spikes and CO2 reactors were not discovered yet
Barely remember that the carpet was often wet because of one reason or another, and if the electrical connections (wires soldered together and "insulated" with band aid) got wet often the 220V made our hands flutter and my mother swear :lol:
More recent lessons:
1) A fancy heater cable will not promote plant growth if you need a chiller for 9 months of the year, and the other 3 months the light bulbs keep the water temperatur cozy.
2) It's easy to DIY a decent hood, but wood is heavy, and there should be some thought put into how to open only part of it for feeding, instead of having to lift the whole thing each time.
3) Need to spend more thought in what gadgets to buy... often they end up unused in the garage. Sometimes it is better to spend more money upfront for some high quality equipment which will last many years, than going through various levels of junk.
4) I need a quarantine tank.
5) High light plants rarely grow well under low light conditions :lol:
I am sure there are many more...