Gravel Vac Triggering A Cycle?

[jmcdonald]

Hello,

This is my first post. I recently found this forum and amazed at the wealth of info here.

I've read various reports on the pros and cons of using a gravel vacuum to pull detritus and waste from the substrate. Some say it pulls out beneficial bacteria, so should only be used to remove surface debris. Others advocate a thorough vacuum of the gravel to pull out as much crud as possible noting that the bacteria are mostly in the filter anyway.

I've got a 10gal. tank that has been running stocked for about 7 years. It once was equipped with an undergravel filter. The filter proved inefficient and so it was "decommissioned" but not removed from underneath the tank substrate. Subsequently, a Whisper mechanical filter and then last month a Rena XP1 were put on. Inhabitants of the tank for the past year or so are (well, until this AM, were) 1 pleco, 2 female swordtails, and an albino rainbow shark.

There's a longer backstory here, but I noted that the nitrate levels in the tank were rather high (over 200ppm) so yesterday I did a 50% water change including a moderately deep gravel vacuum on about a third of the tank's floor area.

Before the water change all of the other readings were spot on normal (ammonia undetectable, nitrite 0, Ph 7.0). It was just the nitrates that were high.

After the water change last night the nitrate had dropped to about 160. Still high, but having been told not to change too much water at once I dosed with AmQuel+ and went to bed planning another water change for this morning.

This morning I awoke to find that two of the fish in the tank had died and the other two weren't looking so happy. I checked the levels: ammonia undetectable, Ph 6.8 (down), nitrate 200 (up-- as I sort of expected). However, depending on which test kit I used the nitrite level was as high as 10! (WAY up!) Nitrite hadn't even registered on either of my two test kits the previous day.

I quickly moved the two survivors to a hospital tank and they seem to have bounced back but the shark and one of the swordtails didn't make it. Sad, really, especially because the shark had been with us for 8 years.

So, it seems like the tank might be cycling, but I can't determine why. A nitrite spike without my test kits showing a trace of ammonia? Usually there's at least some measurable amount of ammonia when the nitrites take off like this in any cycle I've done. So I'm a little confused. Was it the deep vacuuming of the substrate that could have caused this nitrite spike? The deep vac of the gravel in tandem with the old undergravel plates still being in there, perhaps? Could I really have pulled out that much bacteria? And only of the sort that convert nitrite to nitrate? Or is the dose of AmQuel+ messing with my test kits?

Right now the tank is empty so I'm free to recycle, but I'd really like to know what could have triggered this. The deep vacuuming has been done many times before and it never seemed to do anything weird to the chemistry. The 50% water change? My tap water was treated, and does have some trace nitrates (10ppm), but again, nothing new here. Same water, same vacc'ing as always.

I'd be grateful for any input.

Thanks!

James

 

[Sean W.]

..spark notes?

 

[OldMan47]

Welcome to the forum JMcdonald.

A large water change on a neglected tank can give you serious troubles. The rapid drop in mineral content will be hard on any fish present and stirring up detritus from the gravel that ends up in the water can give an ammonia spike. If you have a healthy biofilter, the ammonia will be either nitrites or nitrates by the next day. The usual practice for a tank in that condition is to do several small water changes so that things don't move too fast and of course be careful about what you stir up in the gravel. You have the added complication of an unused UGF that may have who knows what under it. The problem that presents is that the water under that filter plate can circulate back into the tank water and cause an even bigger ammonia spike than most of us would expect from a good gravel vac. Now that there are no fish to be effected, I would drain the water, remove the undergravel filter plate, wash the gravel and set things up again. If you keep your filter full of water and oxygenated during this cleaning, it should be ready to handle the full bioload that existed before your disaster. Your fish in the hospital tank may still have some trouble because they have undergone a huge change in their water chemistry by going from over 200 ppm of nitrates to whatever you hospital tank has in it. If the hospital tank's water also has high nitrates, do not change it too rapidly and they may be OK.