tl:dr - Bunch of fish stuff and opinions and experienced based opinions and such.
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Not sure if this was mentioned already but way back on page 4 (for me) you guys all mentioned stocking. My opinion of stocking is simple: If the fish are healthy & happy, it's ok! Mixing fish from different bioms? Sure, so long as they can live together. Putting goldfish in a tropical tank? Been there, done that, the black moor did lovely until I had to rehome it due to size. 9in pleco in a 75gal tropical community? That's why I have 3 filters and do weekly waterchanges. So long as the fish themselves are healthy and their needs are met, I believe that you can stock as you please. Overstock it a little, that's fine. Push the limits a bit for some fish - well, as long as they don't suffer that's fine too.
This is contrary to many opinions I've found over the years that basically amount to "This fish can't go with that fish because [Insert Generalized Statement Here]. Each fish is unique - I've had betta that were completely passive and tetra that tried to kill everything they encountered - and what kind of tank they can go in depends on the fish themselves, not on a general statement or species-wide generalization.
I've been guilty of this myself, but I do try not to judge people based on having fish of different temperaments or environs in the same aquarium. So long as none of the fish are suffering from it, that's fine in my eyes.
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On to big fish in small tanks! I'm part of like 8 different fish-related Facebook groups, and a year or so ago I saw a post with an 8in fancy goldfish in a 30 gallon tall. There were no decorations, plain white gravel, and a plain blue background. I confronted the guy about it and his response was that the fish was fine in the tank and that it was healthy. I never actually asked myself "is that the fish's future then?" because I hadn't thought of it that way at the time. Looking back, I can honestly say that was a depressing tank to look at, but that the owner truly cared for the fish and took good care of it.
That said, I don't agree entirely with the idea that a fish needs to have as perfect an environment as possible. The fish certainly doesn't know that it's home is anything less than perfect, and so long as it's needs are met (places to hide, space to swim, food to eat, clean water, companions if they need 'em) than it won't live a lesser life, at least, no less a life than a dog lives rather than a wolf. Yes, it's not a perfect imitation of nature, but that isn't necessarily a bad thing - it's more an owner preference. So long as the fish is happy and healthy, the environment it lives in is entirely up to the owner.
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On the Eat or Be Eaten - YES! This. Even if the fish may not live as 'natural' a life as it would in the wild, it will still live a long and healthy life. It's genes may not be passed down, but the chances they would have been passed down in the wild are slim anyway. I cannot bring myself to euthanize a fish, even if it's clearly not doing as well as it could be. Hunchbacks, for example - I have several fish that are hunchbacked. A mosquito fish and my golden dream killi are the ones that haven't passed on after living long, pleasant lives in my tanks. Now, a diseased or dying fish... I still dont like giving up on them, as I've had fish I'd given up on and decided to euthanize "after just one more day" recover fully from their illnesses... but at the same time I've had some that don't. I just don't think that I should kill them - let them fight to the end. Some may disagree. Good for them, but my opinion and thoughts on the matter won't change. I'd rather be alive and crippled than dead.
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Salt. I don't get why people think salt is needed in a freshwater aquarium. I haven't used it since my noob days. I only medicate my fish for things like ich, and I haven't had any illness in my tanks in over 4 years so I don't even have medication anymore xD
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On Advice: I think that the massive amount of advice out there is an inherently good thing. Yeah, not all of it's going to be accurate, but if you look at 20 different sources and average out the information you'll be more likely to have a success than if you look at one source. One source can be faulty - 20 are much less likely to be faulty. One or two in the 20 may be faulty, but overall they'll be accurate. Asking advice on forums or facebook groups is also useful, as the people there who have experience and knowledge can help newer keepers - even if they may disagree on how to help them.
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I think that's everything! This thread was both fun and interesting to read, and I hope my giant post doesn't bother anyone XD