tap water can have nitrate readings in excess of 40ppm before going near a tank. also nitrates will increase the more you feed the fish and on certain fishes bioload on the filter. small tetra type fish have hardly any bioload on a filter especially if not overfed.
as you have been taking your time by adding the stock to the tank which is applaudable as most people just stock a tank outright asap..... take your time to choose what type of fish you want next and add a group of 6-7 of the chosen fish. with what you have already i would be tempted to add either a group of corys or loaches. they will fill the empty space at the back giving your tank movement all over. if you do choose these fish then get all the same species dont mix and match as they will be better off and feel more secure and much more active and fun to watch.
corys and loaches again dont have a massive impact on a filters bioload but feed lightly for the first week and keep an eye on the water parameters for a couple of weeks. i would suggest testing every day for 2 weeks just incase. your 25% weekly water changes are a very good amount and should keep the fish healthy and happy. if the nitrates do start getting a bit high then just do a larger change every now and then. by changing 25% of the water a week you will only be removing 1/4 of the nitrates that have built up and as i said earlier tap water can and probably will contain nitrates any way. mine are at 10ppm the last time i tested so even with a 100% water change my nitrates will read 10. the lower the nitrate the better for any fish but they can tolerate quite high amounts of nitrate and still be fine so dont get too carried away thinking they have to be below 40ppm. also if the tank is well planted depending on the plant types they will also use up some of the nitrates to grow etc.
i would add a group of corys or loaches and review the tank. personally i think that bigger groups of fish look better than lots of smaller groups of different species but each to their own i guess..... maybe adding more rasboras and silver tips would look nice
i lnow we all want lots of colour in our tanks and variety but nothing beats a group of say 40+ cardinal tetras or in your case rasboras