9 times out of 10, the LFS
is NOT a good source of info. I'm lucky, I have a good one.
An aquarium's biological filter depends on the beneficial bacteria, we've already discussed. This converts deadly ammonia into slightly less deadly nitrite, and then into unpleasant, but fairly innocuous nitrate. Like any living organism, these bacteria need food to survive.
With a "normal" setup, you feed a little every day, the fish poop a little every day, the bacteria eat the poop every day. But with a predator tank, especially fully grown preds, you don't feed very often. Sometimes as little as once a week. So, for several days, nothing at all is feeding the biological filter. Then in one day, massive fish poop, and it drops twice what the filter can handle.
I'm not saying you cannot have a small predator tank, just that it requires some very special consideration. What the LFS
guy *probably meant* is that if you forget to feed a fish that needs to eat twice daily, the problems are larger than forgetting to feed a fish that can go weeks between meals.
Re: Keeping fish in small tanks until they grow. Very very very common misconception is that "a fish will grow to the size of it's environment" and be perfectly ok. There's a lot of biochemistry involved, but rather than bore everyone to tears, let's sum it up.
If an adult fish requires at least 50 gallons and 4 feet of swimming space, then a baby of that species requires THE EXACT SAME amount and size.
And absolutely 30 for a dwarf! The smallest dwarf lion can reach 5 inches in length, but most dwarf lions top out at about 8 or 9. Doesn't sound dwarf? The volitans lion (standard size) gets a foot and a half long. A 1 foot by 2 foot tank is just not enough space, even for a dwarf.
Anytime a LFS
employee says "Oh it'll be fine for now" or "This expensive one here is the easiest" or really any bold statement that he/she cannot back up with facts, numbers, or sources.... it's a lie. He or she is trying to make a sale.