If you have bought a 50l tank, there is no point in waiting to cycling it fishless while your moor is in the 20l. You are much better off moving the fish to the 50 l now, and cycle that with the fish in. It is much easier to keep the fish alive during cycling with a bigger tank. <Move everything from the small tank to the large - filter, decor, everything ... that will transfer any bio-filter that had developed so far & might save you a few days or weeks of cycling.>
Doing fishless cycling is meant to spare the fish the ordeal of the cycling process. But if your fish is sitting in an uncycled 5, it is being stressed way more in a small tank waiting for the big tank to cycle. Much better to move the fish to the big tank & do a proper fishy cycle.
Yes, fancy golds can & do eat the white clouds, esp. if the WC are tiny 1/2" frys. However, they do not get sick from eating a WC ... small fish & bugs are normal diet for a goldy.
When a fish is lethargic like yours, the first thing to suspect is toxic water. In a small tank that is not cycled, that is highly likely. It is imperative that you test the water & see what is going on, and a liquid test kit is needed as the stripes are not at all accurate. In lieu of tests, I would be doing a 50 % pwc daily on spec. Clean water never hurts.
Next, you need to examine the fish. Look carefully for:
1. Any spots, dots, bumps, etc.
2. the condition of the slime coat - amount, thickness & color (it should be clear & colorless)
3. Color of the gills - look at the fish from behind, you should get a glimpse of the gills as the fish exhale & the gill cover opens.
4. Attitude of the fins - are they held out, is the dorsal erect?
5. Is the abdomen bloated, are the scales sticking out?
6. Is the fish eating? What is the poop like?
That was the observation part of the physical. You can also touch & examine the fish if needed. This is the full instructions:
DIAGNOSTIC TECHNIQUES
For now, first thing to do is the pwc & water test. See how the fish respond to that & we can take it from there.