Most fish can survive for a week or so without food, but will quickly die from *:
- excessive ammonia levels vs pH and temperature, or
- nitrite shock.
With your system monitoring, be safer and always consider the worst case; your pH is 8.0, your ammonia is 1.5 and you're still on the limits of ammonia toxicity. You MUST measure your water temperature and check your position versus THE chart! (Print and laminate this chart and keep it with your test kit!)
The chart does not take into account all factors, so should be used as a general guide to ammonia toxicity levels; it seems you've been sailing very close to the wind and probably already exceeded the charted thresholds, but it's always better to be safe than sorry.
On April 16 your ammonia level was at 1, then on April 17 it was 2 (only today falling to 1.5). Had you been surreptitiously feeding the fish? If not, what do you think caused the ammonia to rise on 17/18 April when you weren't not supposed to be adding anything to the system and continuing to exchange your pond water? (The answer might have to do with stocking density in an immature system? Or, were there other factors that we did not see from this vantage?)Nitrites
There isn't a definite toxic level of which I am aware, but "too much" will kill fish so we try to keep the nitrites as low as possible. Some people mitigate against nitrite distress by adding salt, but "too much" salt will also kill freshwater fish! Consider slowly adding salt up to 3ppt (up to 3kg of pool salt per 1000L of water, added over a few days) to alleviate nitrite distress now, and maintaining about 1ppt (1kg of pool salt per 1000L of water) for general fish health later (unless you're growing strawberries which HATE salt, but for now the welfare of the fish has priority over the plant welfare).Nitrates
Fish will survive surprisingly high nitrate levels, someone once mentioned to me their highest was in excess of 500ppm as tested through a laboratory, without any fish deaths. The levels detectable using the standard water test kits would generally be considered safe for fish in most circumstances. "Excessive" nitrates can cause other adverse reactions such as algal blooms, so if your nitrates are "high" simply add more plants.pH
Each system is different and will find their own equilibrium point, however, before that, when a system is cycling for the first couple of times there might be a sudden but dramatic drop or spike in pH. As your pH is already caustic I'd be preparing to buffer against a sudden drop in pH to the acidic side of neutral. Add a cup of shell grits to the system (I have mine in the FT, bit some prefer to scatter it in their GB, I don't think it makes much difference?) If the water is acidic the shells will dissolve; if the water is caustic they wont dissolve hence their use as a buffer. (Also, shells are mainly calcium which is an essential element for plant nutrition so adding them to your system will aid healthy plant growth anyway.)For now
An old saying, "Dilution is the pollution solution!"
We're trying to take control of the ammonia and nitrite levels: As has already been mentioned in this thread, keep replacing about 1/3 of the water per day (slowly over the day) with clean, fresh, degassed water of very similar pH and temperature to avoid shocking the fish, avoid feeding or adding any more nutrient to the system until the beneficial bacteria have fully colonised and are able to 'eat' all the available ammonia and nitrites. For the fact the ammonia levels are dropping whilst the nitrites and nitrate levels are increasing means there are some beneficial bacteria in your system, but probably not enough yet to deal with the loads you have on them. You need to allow the little bacteria critters to complete the work you have already given them, and to increase their populations. Have patience, and what ever you do or what ever changes you make, do it slowly. Any rapid changes might have dire consequences for the stability of the entire system.Looking ahead
When you get to zero ammonia, zero nitrites with a smattering of nitrates, don't go silly and feed the fish a bucket load of food; give them a pinch to eat and monitor your water closely for the next 24 hours. (With your fish stock density you might not ever get zero ammonia because the fish are still respiring and discharging wastes, even without feeding. If you chart your progress you might discover your 'zero' ammonia level is 0.10, 0.25 or something other than zero.) After 24 hours, if your levels are again "zero", 0 and 0+, then you can feed the fish again, maybe a little more than last time, then monitor the water again. If there is "excess" ammonia present, make a very careful decision about feeding the fish.
When your system can handle the ammonia produced as a result of one full feeding per day, you can try two smaller feedings per day and increasing the feeding size gradually until your system can handle two full feeds per day, then three smaller feedings etc. Some people might prefer instead to start with three or four very small feedings per day and very slowly increasing the amount of food at each feeding, monitoring all aspects very closely. Each way has its merits.
I'd still be looking to 'thin the herd' and reducing the stocking density until the system is more mature. Maybe there are other members near you with spare capacity who could agist or buy some of your fish? No one learns to walk with a marathon, start small and be safe.
* From practical experience, fish also die quickly from:
- chloromine and/or excessive exposure to chlorinated tap water (Father-in-Law left the mains water hose running into the FT)
- lack of water / exposure to "too much" air (piping failure pumped all the water out of FT)
- next door neighbour herbicides / chemicals (lack of cover and educating neighbours about spray-drift)
- electric shock (faulty pump)
- crows pinching fish out of FT (even through 25mm mesh)
- ike jime (a spike through the brain at harvest time!)
2008-2009: Experimental, NFT
2010-2012: IBC system(s)
2012-Now : 1000L FT, 500L GB, CF, no additional filters