I don't know the proper name, I think it my be salt and pepper Cory Dora, I ended up naming them salt and pepper because of that, just restraint the isolation again, in a qt tank with not heater the tank could easily reAch temperatures of below 10degrees Celsius during the night, do you still think it's a good idea, seeing as it is winter here in Australia?
Ooh yeah, forgot about winter there. When it's summer here, I sure wish I was in Australia. O-kay. If you put him in a QT container small enough, you could always float it in the tank, so long as you're sure he won't try and jump out or get distressed. Otherwise I'm afraid a second heater may be the only other option.
Salt-and-pepper . . . are they very very small? There are two types of cory cats that could fit. One is the Peppered Corydoras, which is among the larger cats and can get up to 8 cm, although they're usually smaller. The other type is commonly called the Salt-and-Pepper Corydoras but is also known by its scientific name c. habrosus. They are "pygmy" corys and very small, only reaching 4 cm at the very maximum. They'd be close to the same size of your tetras. If that's the case, you can get maybe four more of those cats. If they are the Peppered Corys and are big, then you can get one or two more because they'll be happier in a group.
Just getting back to Freddy, I noticed he sort of "sinks" back down to the bottom, like he's sort of swimming up but can't quite do it, he can go to the top of the tank for food and some air, but he seems to go up and "sink" back down for a few minutes and then come back up...
Could it be a swim bladder disease? I know it can be cause by rapid temperature change, like when the neon died.
It could be . . . swimbladder disease is often accompanied by huge bloat, though. Often what happens is they bloat up and that causes their organs to push against their swimbladder. If the bloat can be reversed, usually so can damage to the swimbladder but not always. Damage to the swimbladder without bloating can be caused by injuries, like if he was being rammed in the side by something (another fish or he ran into a decoration etc). Hmm. Can he stay upright or does he have a tendency to tip over on his side while swimming?
When he lies down in the bottom he is mostly on his side, when hes swimming he's usually quite straight, sometimes if he doesn't lie on the bottom of the tank for long enough he sort of tries to swim up, and does a vertical "u-turn"
I've also given him half a frozen pea, as a treat and to see if it helps him over the next few days. He really liked it :)
A little bit of pea is good once a week because of its fibre, helps keep his little system moving; bettas are prone to constipation for some reason. Plus, as you noted, they love 'em. As a tip, you can blanch it first to make it nice and soft for him. Hopefully his difficulty swimming is from shock at temperature change. If he doesn't improve or he gets bloated, then something else may be going on.