90Gallon tank and fish situation

Question
Hi there! I was hoping you could help me out. :)
I've had saltwater tanks for several years now, and recently had an unfortunate situation happen. After 1-2 years of super healthy fish, I went from one morning with all fish eating and acting fine to that same night 4/5 fish dead, and the last one struggling but he survived. Only thing I noticed is faster breathing and PH a bit low possibly due to overfeeding, so was going to do another water change (just did one 3 weeks ago). Otherwise everything was normal!

Here are/were my stats:
- 90gallon fish only tank
- Clown trigger (3-4"), Dogface puffer (2-3"), Emperor Angel (3-4"), banner (3"), Lionfish (5-6")
- Ammonia - basically 0
- PH was lower - 7.9 (I was about to change water to adjust)
- Salinity - 1.025
- Nitrate - can't remember (at work, but when back home will read it again, was in 'acceptable' range)
- Nitrite - 0.05 at most
- Temp - 80-82F (keep it on warmer end)
- System - backpack protein skimmer, Fluval FX5 400g filter, UV light, chemipure, nitromax
- 70+ pounds of live rock (cycled for 3+ years now)

I've spoken to some folks and they've told me its due to a water crash - totally get the water conditioning issue given the PH etc. But they've been through worse! They had a fluke outbreak, pretty bad, treated with Praxipro and was totally fine. I know that if you overfeed them a bit even after a water change, the fish can create an overflow of nitrite / waste which suddenly spikes super fast and then they die. Then its over and goes back to normal basically, so impossible to really plan for nor test properly (and I work during the day so cannot test throughout the day). And it happened so fast! From showing NO signs of problems besides the breathing, they were eating fine and no loss of color. Then suddenly 4/5 dead! They also think my lionfish could have poisoned the tank due to stress (also since he's on the bigger end). Best way to avoid of course is frequent water changes, fewer fish (especially when you have big waste guys like I did), bigger tank, and avoid overfeeding - keep feeding smaller not just 1 big time per day. This situation has also happened to me 2 years ago, basically same situation (included lionfish) but not as many deaths.

So now I have just the lionfish and he's totally fine and doing great. Levels are perfect, Ph back to normal and all dandy but still missing my 4 awesome fish. :( I don't want to go through this again coz it really sucks of course. And I'm being careful!

So now I'm thinking of keeping at most 1 aggressive fish in the tank: Lionfish, or sell him and get a porcupine puffer (always loved) or an emperor angel. So just 1 fish in the 90 gallon tank. Is this enough for cycling?

Would love your feedback on all of this! Sorry so long :)
Thanks!
Francesca

Answer
Hey Francesca,

sorry to hear about your loss.

I dotn have much of a guess, but would like to know if you really know what the water temperatuure is during the day when you arent home?

Could it be hotter while you are away and maybe cool down a little when you get home?

If so, you might be running to close to the edge with water temperature and how it is related to O2, (oxygen), in the water.  the hotter the water the less O2 in the water.


Also, the pH issue is not a big isue.  that is not too low.  My pH will fluxuate from that to 8.3 sometimes...

I would look at your cannister filter and make sure it is totally clean.  If you have the bio-beads or balls or ceramic whatevers, then i would lose those...

Replace all the sponges, (I replace them every month or two when i had a cannister)

If you can, place some fans around the tank/sump to blow across the water and keep the water temp cooler.

sorry i couldnt help more and would love to prevent this from coming back but guessing thru a list of potentials would not b helpful...

good luck and please keep me posted


Bill