Please share your knowledge

[CatsandOatcakes]

Hello. I've been keeping fish for a couple of years since I got a tank for a gift. The problem is that that they only Iive for 4 to 6 months. I'm trying to do the right things (obvs not very well) feel awful for any fishy deaths. I am a poor fish parent.
It's a 60 litre tank and I do a 20% water change each week using bottled water as the quality here is poor. Have 6 fish in. have had 8 or 9 at most. Also have live plants.

Do you have a filter in the tank? If so how often are you cleaning your filter?
Do you have a heater?
What type of fish are they & how many of each type?
Do you test your water for ammonia and nitrates?
How often and how much do you feed?

Yes. Filter changed monthly.
Heater on 25
1 red finned shark
2 platy
2 Danio
1 tetra
Feed the shark 5 sinking pellets once a day.
The rest get flakes but in reality they all share.
Tested the water last week and no ammonia or nitrites and the nitrates were under 40.

The tank is a 64 litre one. Pets at home one complete with filter and heater.

I wouldn't be using bottled water, if your tap water is not good enough after using dechlorinator I would use RO water, bottled water often has added vitamins etc which can cause problems for fish.

Yeah. I started using the bottled as the nitrates are at 28 when it comes from the tap. Where do I get RO water from and is it costly?

Most fish shops, not sure about places like pets at home, not sure was we use tap water.