The easy balance - don't bother. Do the water changes. I don't like things that give the impression that all water changes do is remove nitrate. There are other things in the water that need removing on a regular (ie weekly) basis. Things that are secreted by the fish and live plants. And the water changes will also get rid of the fish waste caught in the gravel (assuming you have gravel
) because water changes are done with a siphon tube with a wide end which is pushed into the gravel to suck up the muck. Did you get one of those with the tank?
The filter is fine for a 60 litre. Though the thing that does worry me is the "quite new". This could mean it isn't even cycled - that means it might not have grown the two colonies of bacteria I mentioned earlier. Or not enough bacteria for the fish you have.
Pets at home do sell the API test kit that I have (the one in the link I gave you) but it is £31 (and that's the reduced price on their website)
If you can't run to that just at the moment, get the ammonia and the nitrite testers (they are only £9 each on the website). The big set contains pH and nitrate testers as well, but they are not as important just yet. Don't believe any shop that says you don't need one.
In fact lesson #1 for new fishkeepers - don't believe
anything a shop says until you've researched it for yourself.
As for your fish, let them get settled first then think about what you want to do.
The angel will grow too big - they need at least 18 inches depth of water or they do a 'jaws' impression and their fins get deformed. Min tank size 100 x 40 x 50 cm
The clown loach also needs to go. They grow huge (over a foot), need to be in a shoal of at least 6 and a tank with a footprint of at least 180 x 60cm.
The plec could be anything from a 4 inch bristlenose to a 2 foot sailfin. As Colin said, there are no plecs suitable for a 60 litre.
I'm curious about the gouramis - you said you thought they were
sparkling gouramis. Are you totally sure? If they are, you could have a group of them, but if they are something bigger (such as an
opaline) they need a bigger tank.
The golden barbs and cardinals are fine
These 2 species alone take up two thirds of the amount of fish you can have. The only possible problem is that cardinals need 24 deg C minimum and the barbs like 24 deg maximum. You'll need to keep the tank as near 24 deg as possible.