Ammonia

Nitrate in a healthy tank should be about 5, so don't worry about that.

 

in new aquariums nitrates will gradually climb as the biological filter becomes established.
40ppm or less is recommended for freshwater. If you keep something like say Discus or inverts...then you want it to be much lower (I always aim for 20ppm or less being I keep discus).

Partials/water changes won't always help with nitrate reduction either (some water sources are high in nitrates).
If you run into this issue then you'll want to use something like api NITRA-ZORB (media that will be placed in your filter) or possibly purchase something like an api tap water filter.

 

once the ammonia and nitrites are at 0 then a weekly water change (25 - 50%) with gravel vac should do.

 

with all reading 0 you're still cycling. You should get some sort of nitrate reading. Did you shake the nitrate 2 bottle REALLY REALLY well?

I understand this tank has been setup for 2 years, but for some reason it's going through probably a mini-cycle.

Could be they added something to your water, could be filter was cleaned improperly (ie. simply rinsed in sink instead of removed tank water), etc.

IF you like products that help look for something like Seachem Stability or Tetra SafeStart.

 

it's best to check water parameters before you do a water change. seeing that you did a water change this morning, i'd wait until tomorrow morning (about 24 hrs) to retest to get the best results.

 

For a 2year old tank you shouldn't be encountering this. You're going through a cycle (like when you first start up a tank).

How are you maintenancing your filters/media? What type of media are you using?
What chemicals/additives are you using?
Test your water source and post the results.
Do you have anybody that would have "dirty" filter media or possibly some substrate that you could borrow to help you speed up the cycling?

I know others will chime in, but providing the above information will help them give you advice that WILL help.
This definately explains why you have lost fish/plecos in the past.

You just need to:
1) get this tank properly/fully cycled (articles on how to do this were linked previously)

2) do routine partials using a siphon. Always drain water from the BOTTOM of your tank. Also be sure IF you have substrate to be sure to "vac." it. This will prevent harmful anaerobic pockets/dead spots/waste buildup.

*Be sure to add your dechlorinator BEFORE adding water back to the main tank. Depending on which product you use it MAY be safe to dose for the entire tank volume just to be on the safe side. (I do this for my 90g using Seachem Prime...even if I remove 50% I dose for the full volume size).

How often should I do water changes?
some will suggest to do them when your test results show the need, some will say do it on a weekly/monthly basis once your cycled so you won't forget, blah blah blah.

3) do filter maint. monthly or as suggested by your filter mfr. (not a good idea to do at the same time your planning on doing a partial).
Directions-->get yourself a 5 gallon bucket and label it "fish" and only use it for things fish related. drain about a bucket of "dirty" water into your bucket and "swish"/"clean" any filter media in it. Then promptly return your media back to your filter.
I know to some this doesn't make sense...but to fishkeepers this maintains your beneficial bacteria which helps eat the "baddies".

-->*Please note that there are a select few types of media that will read that it is okay to rinse it under running water..if it says this then it's perfectly fine to do so; just be sure to add a bit of dechlorinator to your tank if you choose to do so.

 

any reading of ammonia is lethal. if you can't do a water change dose it with Prime for the tanks volume to render it harmless for the time being.

 

test your tapwater, if you have any media in another system squeeze it into this one (ie. sponge filter).

 

Agree with Dawn, have you tested your tap for ammonia?