The stomach is the softest part of most animals and usually where other creatures start eating it.
If you can post some pictures of the remaining tetras, I can check them for disease.
The angelfish appears to have a cuople of white patches on its side. This needs to be monitored closely and could be from poor water quality.
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It doesn't matter how big a tank is, they all take the same time (4-6 weeks) to cycle (develop the beneficial filter bacteria). A bigger tank is better because it has more water and any ammonia produced by the fish is diluted in the larger volume. So if they are in a 30 gallon tank, that is better than a 10 gallon tank.
When doing a fish in cycle, you feed the fish 2-3 times per week and do a 75% water change and gravel clean the substrate 4-8 hours after feeding. This dilutes any ammonia produced by the food and fish.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
You can buy bottles of filter bacteria from most pet shops and these can help speed up the cycling process. Add a double dose every day for a week and then pour the remaining contents into the tank. Try to add the bacteria supplement near the filter intake so the bacteria get drawn into the filter where they belong.
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Don't clean a new filter until it is 8 weeks old. If you clean it too early you can upset the filter bacteria and restart the cycling process. If the filter starts to block up, you can clean it but do it gently and don't replace any media/ materials.
When the filter has established it should be cleaned at least once a month and every 2 weeks is great. Wash the filter materials/ media in a bucket of tank water and re-use them. Tip the bucket of dirty water on the garden/ lawn.
If you have filter pads/ cartridges in the filter, these can be squeezed out in a bucket of tank water and re-used until they start to fall apart. At which time you replace them with a sponge from another type of filter. You can use a pair of scissors to cut sponges to fit your filter. Sponges will last for years and don't need replacing until they start to fall apart.
If you do have pads/ cartridges, it's a good idea to add a sponge now so it can develop the beneficial bacteria too. If you want to remove the filter pads, then have a sponge in the filter (next to the pad) for at least one month before you remove the pad. If you have several filter pads, remove one and replace it with sponge, wait a couple of months and remove the other pad and replace it with sponge.