New Tank

That sounds like a bit of a shock 

Fish should be fed the amount of food equal to an eye - a fish's eye, with a few exceptions, is the same size as its stomach. So all you have to do is measure out as much food as equal to one eye for each of the fish you have. Yes, I do know that's impossible  but it gives you some idea.

What did the previous owner feed the fish compared to what you use? Small fish are usually fed on flake but larger fish might be used to pellets. Did he give you their food as well? A lot of fish take a while to realise that different food is actually food.

Do you know what species the silver tetras and the black fish are? Just to check so you know if they need any special food.

Penguin tetras and mollies will be fine on general fish food. There are dalmation mollies though they are usually white with black spots (like dalmation dogs!)

If your nitrate is 12.5 that is perfectly acceptable. Some people have 40mg/l (or ppm, both are the same) in their tapwater. The ideal is to keep the nitrate level from going higher than the tapwater level plus 20 just before a water change. When measuring nitrate you should always take the tapwater level into consideration as that's the lowest the tank can be unless you do something to lower it such as using special filters, have lots of live plants or use reverse osmosis or deionised water. Nitrate is only high enough to worry about when it gets into the hundreds.

What colour are the angel eggs? Sometimes a pair of females will lay eggs but obviously they'd be infertile. Infertile eggs are opaque white. If yours - well most of them - are not opaque white, they are fertile and you have a male and female. Some of their eggs will be infertile and opaque. Two words of warning - cichlids often eat their first few batches of eggs till they get the hang of it. Don't be surprised if the eggs disappear. And a mating pair of angels could well herd all the other fish into one corner of the tank to protect their spawning site.

If you can remove the eggs it would help stop the aggression. Though if they are infertile, they will eat them. They usually pick out the odd infertile ones from between the fertile ones.

That food is fine. Rather than feed that every day, try giving live food once or twice a week as a treat. Frozen live food is better than real live (less chance of disease) and dried live (not much nutritional value left). One block contains a surprising amount so cut it up before thawing in a bit of water. Most fish go for bloodworm, daphnia or brine shrimp.