I have a 250 L tank with community fish mainly tetras, danios, platys and a couple of swordtails. I have about 30 in total. I want to get some more fish of similar size. How many more fish could this tank safely take?

they're all reasonably small fish so as long as the tank is well aerated and plenty of place for them to hide around eg plants, rocks and driftwood to make the feel safe and not stressed u should be able to have a lot more.

my 3 foot which is just over 200L has at the moment 15 zebra danio, 15 pearl danio, 10 young kribs, fire eel, 2 breeding pair of convict one striped one white [trying very hard to catch these out as they're creating havoc :roll: ] which means hundreds of frys, breeding pair of BN and some random guppies and killies.

so what else do u want in ur tank?

i want to get some more of the same, tetras, swordtails and platys nothing any bigger.

My 255L planted has 10 cardinal tetra, 5 sterbai cories, 2 whiptails, 2 SAEs, 3 dwarf chain loaches, 7~ 10cm discus, 2 blue rams, 4 ottos, and 1 queen arab pleco. Nitrates and everything else is low, but I have 2 external filters running and twice weekly water changes.

http://www.practicalfishkeeping.co.uk/pfk/pages/stocking_density_calculator.php

Heres a cool calculator, but its just a guide.

There are many points to take under consideration when deciding on stocking levels. I prefer to keep it on the low side so if there is a power cut, or something goes wrong, things don't go so bad so fast.

The better your filtration and aeration, the more plants and the more water changes you do regularly all help you keep higher stock levels but if only one thing goes wrong (heaterstat dies, filter stops, you hurt your back and are unable to do a water change etc) then the whole thing can get very toxic very quickly.