It's better for the fish if you can do a water change more frequently. I normally recommend a 75% water change and gravel clean the substrate once a week. If water is limited then a water change every 2 weeks is better than once a month.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Try to gravel clean any open sections of the gravel whenever you do a water change. You don't have to gravel clean the substrate where the plants are. Just come out a couple of inches from the base of the plants and work around them
----------
Beneficial filter bacteria takes about 4-6 weeks to established. If you are replacing filter pads every 2 weeks, you will be removing the beneficial bacteria and messing up the filter cycle.
Filter pads/ cartridges/ materials should only be replaced when they start to fall apart. If you have pads/ cartridges then add some sponge to the filter and leave it to run with the pad for a couple of months. Then chuck the pad and leave the sponge in the filter. Sponges last for 10+ years.
Sponges and other filter media get cleaned by squeezing or washing in a bucket of tank water and the media is re-used. The bucket of dirty water is poured on the garden/ lawn.
Carbon isn't necessary in an aquarium unless you have heavy metals or chemicals in the tap water. Then you should filter the tap water through carbon before adding it to the tank.
You can replace the carbon with another sponge for more filtration.
----------
Fish should get a varied diet and not fed on one type of food. You can buy various fish foods form pet shops or make/ grow your own. You can use raw/ cooked prawn, fish, squid and cut them into little bits and offer a few pieces at a time.
If you have aphids on roses in the garden, you can tap them into a bucket and offer them to the fish. Small ants, ant eggs and small termites are eaten by most fish, as are flies and mozzies caught without bug sprays.
If you have a bucket of water outside in the garden you might get mozzie larvae in it. These can be scooped out with a net, rinsed under tap water and fed to the fish.
You can buy Daphnia from some pet shops and culture them in a large plastic storage container containing green water. You can also collect Daphnia form freshwater ponds in the southern half of the country during autumn, winter and spring.
You can buy live brineshrimp from most pet shops and feed them to the fish.
The following link has information on culturing live foods for baby fish. It explains how to make green water and culture Daphnia if you're interested.
STARTING OUT Make sure you have a pair, (1 male + 1 female). There is nothing worse than spending your hard-earned dollars buying a couple of fish to try and breed and ending up with a pair of fish of the same sex. Let's face it, to reproduce fish you need a male and a female. Two males won't do...
www.fishforums.net
----------
I would get rid of the silver dollars and possibly the Congo tetras, and then increase the number of other fish (black neon tetras, red phantom and rummynose tetras) so you have groups of 10 or so of each species of tetra.
However, if you have severe water restrictions and can only do a water change once a month, then get rid of the silver dollars and Congo tetras and maybe get 6 more black neons and rummynose.