Hi, Army Reservist/Personal Trainer/Dad here...
Some good points from other commenters but let's just look quickly at what muscle tone actually is. It's your muscle fibres twitching and tensing. What happens when you exercise your muscles is you tear the fibres. Your body then rebuilds the fibres stronger and more elastic than before which is how your muscles grow. It also comes from the blood pumping close to the skin from your blood vessels (which is why the veins appear to pop out).
However for the tone to really show off (in an Emily Blunt in Edge of Tomorrow kind of way) you need a fairly low body fat percentage.
Since you can't target any specific part of your body to burn fat you'll just have to burn it generally. Good news is you can do this though resistance training as well as cardio.
Whilst there is an argument for repping close to your 1RM (1 rep max - the most weight you can lift in one go) it isn't going to be effective for what you want to achieve.
Repping high weight is great for overall strength and power but it isn't great for increasing your muscle endurance and quick twitch fibres (what makes you go faster). You want the latter to increase tone and not bulk (although like others have said bulking is very very hard work).
Just look at a strongman compared to say an American Football player or a prop forward in rugby compared to a winger. One needs to be strong and powerful. The other needs to be fast and have strength endurance. Which one looks more toned?
So the solution? Pretty much along the same lines you were doing...just more. You have identified a weight you can manage for high reps.
Instead of 4 sets of 20, you want to be aiming for 10 sets of 10. For example; (obviously mix this up how you wish - don't neglect body weight exercises as others have said - however I disagree with what some have said by doing solely compound exercises. You want a mix of isolation and compound* see why at the end)
10 x 10 bicep curls
10 x 10 shoulder press
10 x 10 chest press
10 x 10 pectoral fly
10 x 10 skull crushers
10 x 10 side lat raises
10 x 10 press ups
10 x 10 hammer curls
10 x 10 upright rows
10 x 10 front lat raises
For a total of 1000 arm exercises. Rest 20-30 seconds (Depending on fitness levels) between each set of 10. Rest 45 seconds between each completion of 100 reps.
Alternatively you can try a pyramid;
Pick an exercise - e.g. barbell curl
30 x 30kg - rest 45 secs
35 x 25kg - rest 45 secs
40 x 20kg - rest 45 secs
45 x 15kg - rest 45 secs
50 x 10kg - rest 45 secs
Rest 60 seconds. Then repeat the pyramid in reverse.
For cardio focus on sprints. If you're using a treadmill or exercise bike, set the gradient for at least and 8% climb. Then go for 30 seconds at a time on a high speed. For me I run at 10mph on a 10% gradient.
It's actually surprisingly effective at burning fat and increasing your ability to sprint. You can supplement it with longer fast runs. In the Army we run 1.5 mile dashes in our annual tests. To achieve a respectable time of 9mins30 or less you need to average close to 10mph.
That said your aqua aerobics is probably getting enough in for you and will do wonders for your heart and core.
So in summary you can tone up using high weight and less reps but you're more likely to build overall strength and power since you aren't really tiring your muscles out from an endurance perspective. However it is far more effective to overload your muscles through a higher rep count which will increase your fast twitch muscle fibres and the blood pumped around your body thus increasing the appearance of tone.
Like I said you can see it in practice - compare an olympic sprinter and a hammer thrower.
*so the reason you want to mix isolation and compound is mainly to just get a better workout for your big muscle. For every big muscle you have loads of little and big support muscles. During a hammer curl you also use your shoulders and chest. This is why compound is great to increase your stability and coordination as well as strength etc. However sometimes smaller muscles won't get worked out so much which is why you need isolation exercises.
For example your tricep is made up of the long head, lateral head and medial head. Different exercises target different muscles and you can isolate different parts. For example a tricep dip will work mainly your long and lateral head but not your medial head as much. A skull crusher will work your medial head more than the long and lateral.