Freshwater / Tropical fish dilemma!

Hi..

[...21 litre tank...]

1: In my mind almost too small for the most - even dwarf -
fish species..

2: Please try to avoid hectic wanderers like some rasbora,
brachydanio..

3: Add some MTS. Snails are able to transform dead food,
plants or fish into excellent nitrifying bacteria food..

4: What's about lightening and water parameters..?

a) Any suggestions for a good starter freshwater fish that
is pretty durable and won't grow to a huge size? I want
something quite pretty so am prepared to pay a bit over
the usual bog-standard prices....

5: Some tiny colourfull killi fish?

b) I'm considering buying a heater and getting some tetras
or tropical fish: is this advisable for my first proper
tank?

6: Yes. In the 60s were 5W and 10W permanent heaters and
low flow filters available for smaller tanks. Missed them
later in the pet fish stores but they staged a comeback
in the *Nano* world..

7: Would recommend a 10W thermostat heater and a filter with
maximum 60 l/h (15 g/h) output..

I was trying to think of the extra hassles that
heated tanks would bring, but can't really think of any.
What are the pros and cons?

8: I suppose for a beginner a tropical tank were easier to
set up and to manage. A lot of colourful tropical _dwarf_
fish are available.

I assume you can add cold
water during 10% water changes....

9: Well, where I come from tap water is drinking water so
I'm used to recommend water changes of 30-60%. But see no
problem to warm treated tap water up to room temperature in
a bucket or so, or you might even stick a small thermostat
heater in that bucket. Feel challenged to be
creative..! :-)

Any ideas would be much appreciated - determined to get it
right this time!

Have fun and enjoy..!

--
cu
Marco