I know this is a rather old thread, but let me give some input anyway. 

Water changes= The recommended amount of water to change in a system is 20-30% no less than once a month. In the case of goldfish you can do it weekly or bi-weekly. Doing water changes of a higher percentage can change the waters parameters too quickly (temperature, chemistry etc.) and cause shock. In some cases 50% will have no ill effect, if your water source does not differ from your tanks conditions too much.

Conditioner= I have used plenty of different brands over the years and my favorite is Prime by Seachen.
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Air bubbles and canister filters: The reason you want to keep the air bubbles away from your intake is that most canister filters are powered by top mounted motors. Any air that gets into them can cause an air pocket around the impeller leading to a short life or malfunction. Some newer models have included a purge system to counter this but it will still cause the filter to be less efficient.

Magnum 350: this is one of the few, and I mean few, canisters that do not suffer from the above phenomenon. This is due to the fact that the motor is located at the bottom of the canister and it pushes the water rather than sucking it. Magnums are good filters, but you have to know what they are and how they function. The filter itself does not contain any bio media. It is a straight mechanical/chemical filtration system. It has to have either biowheels or a fluidized bed connected to it in order to provide bio filtration. When coupled with bio wheels or a fluidized bed they are excellent filters. They have zero water bypass which makes them more effective than most canisters, but this can lead to the collapsing of the basket that was described. The reason you don't see that collapsing in other filters is that once they get too clogged the water just starts to go around the filter. A Magnum should not be noisy, if it is then you more than likely have a leak somewhere allowing air the system faster than it can purge it. The major advantage to a Magnum is that it can run a surface extractor without any fear of burning up your pump due to getting air in the filter. Another advantage is the use of micron filters. You want nice clean polished water, throw one of those puppies in your Magnum, but be ware that you have to clean it often as it will get clogged.

In the case of goldfish that's fine. But try doing a water change that heavy on a tropheus tank and they will be belly up before you eat dinner.   I keep forgetting that this site is for "goldfish".

Nossie, do you have pictures of your tanks/fish... I would so love to see them.

Wow I never knew you had all these fish!

I think it might be aluminum? But I'm not entirely sure, the metal parts are pretty light-weight
It's been custom made by a company here within the country, so this is a very common sort of tank over here (Those with built in cabinets etc are ridiculously expensive!!)

fantailer: But as far as I know you've been checking those threads out every once in a while Oh well, everyone forgets sometimes I'm really going to snatch someone's camera and get a nice photo of it...