you've had some really good advice here, just one thing to note is that you shouldn't believe everything the filter box tells you. they're calculations for tank sizes are always a little optimistic.
really if a filter says it's suitable for a 70l tank, it's probably only suitable for a tank around two thirds that size, so the stingray you have would probably only provide good filtration for a tank around 50l. You shouldn't ever try to push a filter to it's limits, you'd be much more likley for it to fail or to under perform. I do think you need to look at getting an additional filter or upgrading the stingray to something bigger.
IMO there's a handful of basic lessons which if you follow them you'll find fishkeeping relativley easy
1 - Learn the nitrogen cycle and fishless cycle your tank
2 - Always overfilter
3 - More is more when it comes to maintenance, don't skimp on it and it can come back to bite you later
4 - Research fish and stock sensibly for the size of your tank
Don't need to spend a lot of money on expensive gear and medications, just take a little time to get the set up right in the first place and you'll find it all a whole lot easier.