I would have one basket with filter wool and put ceramic noodles in the other 3. You can use gravel as a media but it does not work as well. The ceramic noodles have huge surface areas and promote more bacterial growth (or something equally geeky) :lol:
I would suspect something other than the temp in that case. Guppies are basically pretty hardy and a broken heaterstat would result in a slow temp drop. Hovering at the surface sounds more of a lack of oxygen problem. They will live in temps down to 18C.
Most often you can put things for them to lay on and they will go and lay on everything BUT the thing you wanted them to :lol:
My son's angels have spawned ON the heater a few times :roll:
At this time of year you ought to have no trouble finding live critters in small ponds etc to feed the fish. Check nobody has sprayed nearby or anything though.
Angels get in the mmod anyway with or without live foods.
Most fish will cope with higher temps for short periods of time. It is constant high temps that cause problems. At night the tank temp will drop again.
As long as the shop knows they are going to be bagged for several hours you should be right. If they know that they should put less fish per bag for you.
We regularly bring fish up from ChCh with no deaths so far (you must drive slow and stop a lot as we do the trip in under 4 hours, inc comfort and food stops). We have brought them from further afield and had them bagged for up to 2 days without problems too for that matter.
Go to farm supply stores and get the stuff farmers use in dams. A lot cheaper than the "proper" pond lining.
Do they have any farm supply stores in or around Auckland? :lol: