How many platys in 70 litres (18 US Gallons)?

[April_ht]

I’ve recently gotten a new 70 litre tank which has been cycled and fish were added in earlier today. I’ve got 9 platy fish, ranging from 1.5-2 inches, 5 males and 4 females and 2 small sucking catfish. I know the male to female ratio is off and I’m planning on getting rid of 3+ males soon, and I have yet to see any fighting between males and females.
Is the tank overstocked? Would it be a bad idea to add more platies (or even another type of fish such as barbs or tetras) once a few more platys are gone? Would I need to remove more than 3 platys to add another type of fish? What fish are able to get along with platys and live in the cooler water range?
Only artificial plants are being used as of right now and 40% water changes are done fortnightly.

Any help would be greatly appreciated, as would advice on any problems or issues you find with my tank.

 

[TomW]

I would say 9 is a good amount for a 70 Litre aquarium, as for your other questions ill let someone else answer because I'm not all that experienced yet

 

[AbbeysDad]

I feel that there really is no precise, proper number of fish per gallon of water as long as the quality of the water remains high and the fish are not aggressive with each other. I colony breed swordtails and some would say that my tanks are extremely overstocked. But I use fast growing floating plants, feed properly, and do sufficient volume/frequency partial water changes so the water quality remains high. For a deep dive, see: Filtration/Water Quality.
Having written the above, I suggest that instead of partial water changes of 40% every 2 weeks, you might do 50% (or slightly more) weekly to better ensure high water quality. You might also consider floating plants like water sprite, wisteria, frogbit, etc. to name just a few. Fast growing floating plants grow under nearly any light and are amazing at helping to keep water more pure.

 

[Byron]

You will soon have a serious population explosion if you keep the females. They are certainly all impregnated now, and even removing all the males will mean monthly batches of fry from each female for several months. They cannot possibly eat all of the fry.

Nine platies with male/female is way too many for this small a tank, when (presumably) it is your only tank and you do not have other tanks to take the fry as they appear.

 

[April_ht]

Thank you all for the replies, I will start the higher amounts of water changes more frequently. All the babies we currently have are kept in a smaller tank as of right now and females placed in a separate tank while they had their fry. There’s a good chance not many of the fry will survive as the tank is not heavily planted, only moderated, but fry found in the tank will be moved temporarily into a 50 litre while we re-home them.