Fish deaths, high Nitrates

[Rachsw]

I have a Biorb 30 litre fish tank, I've included a heater (26 C) and it's been running for just over a year. I'm new to fish and this was my first tank so I wasn't too knowledgeable about the amount of fish I could have so up until two months ago it had 2 mollies, 1 platy, 1 guppy and 2 amano shrimp. To my eyes this wasn't overstocked, in fact I thought I was giving them a lot more room than the bare minimum. The water was good and I regularly changed it but then over the course of 1 week they began to drop like flies and I only managed to save 1. The shrimp are still in there, completely unbothered. I thought there might have been an internal bacteria being passed from fish to fish that wasn't affecting the shrimp, but by looking on the internet I realised it was probably because it was overstocked. Since I've put 1 fish in there to test it and it was dead the next morning. I've kept doing water tests with test strips but they were good with no signs of anything wrong. So I went out and bought a liquid test kit yesterday which has shown this: Ph - 9.0, Ammonia - 0.1mg/l, Nitrite - 0.0mg/l, Nitrate - 75mg/l
I'm gathering the amount of Nitrates is really bad as well as the Ph from doing a lot of research in the past few weeks which I should have done when I first got the tank! Basically I'm paranoid of putting any more fish in that tank ever again and want to know what caused the Nitrates to go so high and how to bring them back down. I changed the water 40% today as I haven't since 4 weeks ago. Do I need to do lots of water changes? And does the amount of Nitrates suggest there was lots of Ammonia (which killed the fish) and then that changed into Nitrites and then Nitrates and has stayed that way because the fish are gone and I haven't changed the water until today? Thanks.

 

[Samphotog]

Okay so you're definitely overstocked with your selection. Take a look here AqAdvisor - Intelligent Freshwater Tropical Fish Aquarium Stocking Calculator and Aquarium Tank/Filter Advisor this goes to the AqAdvisor website with your info in it that you gave (Note: You can change any of that if it is incorrect, I had to use the closest thing for the tank size. you could measure it yourself if you want) hopefully this helps.

 

[Fishanatic]

Well, depending if you are cycled or not, it's probably the high amount of ammonia that converted into nitrates, as you rightfully guessed, as I wouldn't think any water supplier would give water with that high nitrates. I'm not surprised you're pH is that high, because it might be from the tap water(recommended drinking water is between ph 9-11 usually), and the ammonia is quite high. I recommend an immediate 90%-100% water change, and once you chose the fish, do some heavy research, because you were overstocked and doing too little water changes. And a pH of 9 is alkaline for almost every fish, if not, every. The water is just too toxic, and you might check into a RO/Distilled water system if this is coming from you're tap water.

 

[Rachsw]

Thanks for the advice, I just learned more about my water in the few minutes it took me to read your responses than I have researching on the internet for hours. I think I understand about overstocking better now and thank you for the link