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Supersquid girl! Glad you've joined our little group here. Anything about squids, octos, cuttlefish and anything else ceph related is a go here. If you look on the home page under articles, there are a number of good articles on ceph care. Squids aren't really kept in captivity, although Dr. Steve O'Shea has had some success in this. Was there anything specific you would like to know about squid care?
While we try to encourage people to learn more about cephs and the keeping of them for education in captivity, squids are for the most part "unkept" for a variety of reasons. If you could obtain a very, very large circular tank (perhaps even an inflatable pool) they could be kept for a while...the biggest problem with maintaining them is that they jet at very high speeds, typically crashing into the sides of their container, and rupturing their insides (resulting in death). Dr. O'Shea and the rest of the squid experts could tell you what size of tank/pool you would require for something like loligo, but I think the total cost of such a project is more likely to be undertaken by a research lab or university.
It would be interesting to hear more from you about your interest in squids, they are fascinating animals.
The answers to all your questions are very much predicated on what species of squid you plan to keep. Since, to my knowledge, squid aren't normally available to the aquarium trade, I wonder if you're planning to get something from your local waters. If so, your number one priority should be keeping the water cold enough - below 12C (54F).
There are two local choices:
1. Opalescent squid (Loligo opalescens) - This fellow is pelagic in nature and quite short lived, even by ceph standards. They eat fish and other squid and probably need too much space to be practical for anything but a large public institution.
2. Stubby squid (Rossia pacifica) - This one is benthic and could probably be kept in an ordinary tank of about 110 liters (30 gallon) with some fine sand or mud since they like to bury themselves during the day. Their favorite food seems to be shrimp but it would likely be perfectly happy to eat tankmates if you put it in a community situation.
Of course, if you plan to somehow obtain a tropical species, everything I've said here is wrong.
P.S. The most economical round tank is probably a polyethylene livestock watering tank. Check with a local farm supply store.