This is a long post so I'm sorry! I have a Fluval Edge 23L tank with one rosy barb and one paradise fish currently. I've had this tank for 2 years, its only recently I've found out that this is way too small for even just one of these guys, let a lone the 2, but this is something I'm going to fix ASAP. (When I bough these guys, I was told I could have 6 fish in the tank by the pet shop I bought it from, how do pet shops get away with saying this?) Anyway, my regular cleaning schedule was 50% water change every 2 weeks, and I rinsed the filter media in the old water. This was working fine until circumstances changed and I had to get someone else to feed them (I moved to another city for uni, couldn't take the fish.) I would still come and do the maintenance every 2 weeks, but I think they were being overfed, I would always find excess food in the tank. I used to have one extra rosy barb, but the little guy passed away while I wasn't around. According to the person who was looking after them, the water went really cloudy a few days before he died (I'm assuming due to the overfeeding and there was a bacterial bloom, and he died from ammonia poisoning?). I wasn't told this, or that I had lost a fish until a WEEK after it had happened (to avoid stressing me out at uni apparently). I test the water as soon as I get back, nitrates were sky high (I did a 70% water change and the API Master Kit was still reading 160ppm). After multiple days of water changes, I got the nitrates down to 10ppm. During this time I also changed the gravel from fine gravel to bigger rocks (this might have been a stupid move but at the time I thought I needed to just clean EVERYTHING). I'm gobsmacked that the 2 fish I have now survived, they both showed late signs of nitrate poisoning. I should also add here that the water had been around 7.6 pH every time I tested it, but I admit I didn't realise the importance of testing aquarium water so frequently, so I only tested the water every 3 or 4 months. I begin to think everything is going to be okay. I start testing the water weekly and start doing water changes weekly too, but I notice the pH of the water is reducing. Ammonia and Nitrate stayed at 0ppm, and nitrate didn't rise above 20ppm before I did a water change. Fish seemed to have recovered, the paradise fish even starts blowing bubble nests again. However, I have a new problem. The pH has now reduced to 6.4ppm. This reduction has been happening since the loss of the fish, (about 1 and a half months ago). I'm concerned as this is a big jump from where it was. The water change I did 2 weeks ago left the water a little cloudier than normal, but I just put it down to bubbles and expect it to settle. I have to leave home for a week and ask the "fish babysitter" to monitor the tank and let me know if something happens. When I get back, the water is still just as cloudy. I test the water and I read an ammonia level of 1ppm. I freak out and do an early 50% water change then wait a few days. The ammonia went down to 0.5ppm, but I don't understand why it is not changing "automatically" and just being turned into nitrite and nitrate. I've done one more water change so I'm currently reading 0.25ppm but I'm scared the cycle of my tank has stopped due to the pH lowering. At the moment, the water still is cloudy, but not as bad. I've been testing the water daily since the last water change but the ammonia is staying at 0.25ppm. I don't know what to do. Should I do another water change, or should I leave it? I don't want ammonia to start building up and risk losing my fish again. They've obviously been though a lot over the past 2 months and I really don't want to stress them any more with bad water. I've attached a picture of the water parameters at the moment (pH is 6.4). I would normally do a water change now at this level of nitrates, but because there is an ammonia reading, I'm worried I'll be doing more harm than good by doing a water change. Should I try to raise the pH? Is this pH level low enough to really affect the nitrogen cycle? Is this all happening due to the fish loss and the stupidly high nitrates I had?