Glad to see other people on this site interested in
Goldfish. I happen to have 2
Goldfish (planning to get 2 more) in a 55 gallon. I'll do my best to answer your questions, but if I can't then I have found the people at thegab.org very helpful for much of the advanced stuff.
You should actually keep that heater. Fancy
Goldfish like a steady temp of about 75F (I only have to run my heater in the winter). The common
Goldfish are more hardy with temp variations, but I do not recommend them for an aquarium. Common
Goldfish really do best in ponds or 100+ gal tanks. With the filter you have you could have about 4 fancy
Goldfish. Fish Profiles: Overview of
Goldfish Types <-- This is a good pictorial guide to
Goldfish breeds. I would stick away from anything without a dorsal. My favorite is the ryukin ^-^. Fantails are pretty but their long flowing fins can become a problem.
Now for your questions:
1+2) Sand is a great substrate for
Goldfish. The substrate needs to be either something small enough that it will pass through the GI tract unimpeded or is too large to fit in their mouths. I've never dealt with sand for substrate; I prefer large river rocks.
Goldfish can also be kept in bare-bottomed tanks. Many people also do that. But I would not use regular aquarium gravel.
3) Yes.
Goldfish love to snack on live plants, but I have found a few species that either outgrow their trimmings or they don't like. I have anarcharis,
Hornwort, and
Water Sprite in my tank. They like the
Water Sprite the most.
Goldfish are also big waste producers and all of the plants in mine are nutrient sinks (great way to keep the nitrates down). A good way to keep them from eating your plants is to provide them with fresh veggies daily; in fact I highly recommend it.
Goldfish can't survive and thrive on commercial feed alone. One way many have found to get around
Goldfish uprooting plants is to plant them in little pots. I haven't had an uprooting problem.
Goldfish are rather varied. Some people have a real problem with issues like nibbling and uprooting; others don't.
4)
Goldfish and snails... I have a troubling history with this. My
Goldfish have eaten almost every snail that can fit into their mouths. If you get one large enough not to be eaten, it will probably survive. Again; it's mostly a case-by-case basis.
5) I can't really answer to this. It depends on what you want. Do you want to watch them grow? Do you want to insure that you have a great specimen?
Goldfish will easily live for 10+ years under the right conditions, so it is not an issue of the amount of time you will have with them.
6) Things to avoid: Putting the eye-deformed
Goldfish like bubble eyes, celestials, and moors together with non-eye deformed breeds like ryukins and fantails. Exercise caution when putting commons and fancies together. Be careful of sharp decorations with most of the fancies. I had a fantail who ripped his beautiful fins on my tank decorations. Another thing to avoid is low-quality commercial foods; these can cause gastro-intestinal bloat which can cause swim bladder disease (sometimes called floaty
Goldfish).
I hope I was of some help! Let me know if there is anything else I can do for you.
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