Hi Leigh,
How are the fish looking today? I was hoping one of our Australian posters might have been able to advise on the treatments you are using, as they are not available where I am, but unfortunately no-one seems to have chipped in so far.
It sounds like a fungal or bacterial problem, or both. Remember that it may take 3 or 4 days after adding the treatment before you start to see some improvement. If there are no signs of whitespot in the tank, it is probably a good idea to stop using the Ick Away, as loaches don't tolerate medications well at the best of times. How often does it state you have to dose with the Fungus Ade? Have you removed any activated carbon from the filter so that it doesn't absorb the medication?
If you have only been carrying out water changes once a month, it would seem unlikely that your nitrates would only be at 5ppm unless you have hardly any fish in the tank, masses of plants, and feed very very sparingly. Could you recheck this, and if it comes out the same, check the expiry date of your test kit and take a water sample to a local store and ask them to double check this for you. Test kits have been known to give faulty readings from time to time.
The problems you are seeing are likely to have been caused by stress from a combination of things - the low temperature, poor water quality/high nitrates (with such infrequent water changes), and then more stress from excessively large water changes when they have been carried out (water chemistry differences and temp fluctuation shock). Anyway, I'm sure by now that you realise loaches benefit from small partial water changes carried out once or twice a week and a steady water temperature between 27 and 29 deg C.
I'm not familiar with the filtration that you are using, but generally speaking, external filters are very good for loach tanks as they tend to have more room inside for filter media, which the good bacteria colonise. When speaking about 'flow', I mean current, as clown loaches are river fish and enjoy a lot of water movement. I would imagine you get reasonable current from the outlet of your filter? Whilst you are treating with medication (and especially now that you have upped the temperature) you need to be sure you provide a decent amount of oxygenation. The aerator (air pump?) is ok, but more decorative than anything - the only oxygen it puts into the water is where the bubbles break at the water's surface. You'd be better off dropping the water level in the tank by an inch or so, so that the water returning from the filter splashes down and creates churning at the water's surface, incorporating a much higher level of oxygen. If you have any powerheads, you can angle the flow diverters upwards, again to cause agitation at the water's surface. This will all help to make your fish much more comfortable.
Good luck, and let us know how you're getting on.
Emma