Guppy Killer?

I set up this tank mid february and truth be told I didn't know anything about cycling and listen to advice instead of doing research.  After a couple of weeks I noticed the fish weren't acting very well so I checked the water, saw there slightly was high ammonia, and changed it again.  At this point I still didn't do any research 'cause I erroneously thought keeping fish was super simple.  Anyway I feel bad about it now but I guess I was poisoning them a bit.  About a month ago, one of our platy's had fry and then almost immediately died.  I checked the ammonia and it was .5.  At this point I started doing weekly water changes and figuring out what was wrong with the fish and added a heater.  SInce then we've been doing weekly, or bi-weekly if we screw something up and the parameters get out of whack, water changes. One of our other guppies looked like she was going to have fry and became really agitated and her gills looked inflamed. I checked and again the ammonia was at .5, nitrites 0, nitrates 20.  I did a 50% water change and she seemed much better even though she did absorb the fry.  That was about two weeks ago. The day before yesterday I noticed some hydra in the tank and I was a little worried as there is a four week old fry in the tank so I got a small sparkling gourami. However before putting him in I noticed that the guppy, the previously sick guppy, had a red splotch on her head, like a blood blister or something that had lifted the scales on her head. I pulled her out and put her in a gallon tub with fresh water and some hornwart. I gave her some food which she did not touch, and since she started to have trouble swimming I gave her a pea, which she pecked at. Today I changed the water and she looked better, however this evening I checked in on her and she was dying.  Her pectoral fins were shredded and had red streaks on them. Unfortunately it is too late to save her, but I would like to know what killed her so my other fish can avoid the same fate.  
 
SO what is my guppy killer? Was it the previous exposure to ammonia and she just didn't get over it? Can small hydra kill adult guppies? Was it some bacteria or something else I don't know about?  
 
I am all ears.  Thanks so much.

Tank size: 20 gallons
pH:7.4
ammonia:0
nitrite:0
nitrate:10
kH:?
gH:?
tank temp: 76*

Volume and Frequency of water changes:
50% once, or twice a week depending on the parameters
Chemical Additives or Media in your tank:
Just conditioner, quiet flow filter
Tank inhabitants:
1 zebra danio, 2 male guppies, 3 female guppies, 1platy, 1 fry that hid so well I didn't see him when I pulled out the others. (There were 4 females and 2 platy at the start of the tank).
Recent additions to your tank (living or decoration):
Added a Sparkling Gourami to kill the hydra, but I didn't put it in the tank until after I pulled the guppy.
Exposure to chemicals:
none
 

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Oh and the tank has a bunch of hornwart, some red wisteria, vallisnaria, and mondo grass

and good on you for admitting your mistakes and willingness to learn.  We have all made mistakes along the way and there are some knowledgeable people on here that will help you.  Good on you also, for you explanation, details and image, which all assist in people helping.
 
 
I do not know a lot about the effects of Ammonia poisoning apart from that it can burn the gills of the fish, so hopefully someone else will come along.
 
My thoughts for consideration are:
Your guppy looks like she may have had a bacterial infection by the way the side of her body has broken down.  If it was bacterial it can sometimes just be the one fish, but keep a close eye on your others for any signs.
Did her fins look frayed at all?  Do your other fishes fins look intact? i.e. is there possibly fin rot occurring?
 
Zebra Danios can be nippy, and are best kept in schools, so having just the one is not advisable.
 
I personally would be doing only a 25-30% Water Change unless there is a high Ammonia/Nitrite spike, which it appears is no longer happening?  In respect to your readings did you take these before or post water change?
 
How do you currently clean your filter? i.e. are you cleaning the filter sponges etc in expelled aquarium water?  What media is in your filter?
 
What water conditioner do you use?
 
I am not familiar with hydra so will let someone else advise, plus correct me if I am wrong
 
Can you add your location into your profile as this assists with different countries having different meds etc.

I would suspect that the exposure to ammonia caused the problem - it's rather like humans smoking, it might not get you straight away, but eventually......
 
It looks as though your filter has now successfully cycled, but what you'd have been best off doing is daily water changes, to keep the ammonia (and nitrite) below 0.25ppm at all times. Let's hope the other fish don't suffer as well.

Hydra can kill guppy fry and everything smaller. The sores on your fish do not look as though hydra was the problem and you guppy would have been to large to be picked on so the most sensible cause would be your ammonia readings.
In future maybe you shall cycle! Also if you have already noticed ill looking fish adding another one to solve the hydra problem was never going to help your ammonia problem.
Ammonia isn't an instant killer as said above but it does look like your fish has 'burning' on the fins, so my guess is the poor blighter was poisoned.

We all make mistakes, at least you're willing to admit your wrong doing and get on with improving your little fishies life!

Keep in mind when you see any ammonia that you should do a large enough water change to make it safe for your fish. If you have a reading of .50 ammonia and only do a 10% water change, you'll still have .45 ammonia. If you did a 50% water change that would bring it to .25, which is still dangerous to your fish.
 
It sounds like you're pretty close to being cycled, so you shouldn't get any more spikes. But be sure when you clean the filter to only rinse the media gently in used tank water you've just taken out of the tank. Don't replace it. What kind of filter and media do you have? 

Thank you for all of your answers!
 
RCA: I don't think the picture shows it very well but its her pectoral fins that are shredded not the side of her body, but yeah I was thinking it may have been bacterial too. Good advice on the Danio, we got it from a friend but it isn't compatible on its own with the tank, I'm sure she'll understand if we give it back. The readings are from two days after the last change, but even before the last change the parameters looked good too. Her pectoral fins were frayed and the fins on the males in the tank were frayed after the first ammonia exposure, but it doesn't look like fin rot, the fins are a bit shredded but instead of white along the ends its dark after the more frequent water changes, they seem fine and their tails haven't eroded anymore.  I just read about adding aquarium salt to the tank to help their tails get back to normal. The first cleaning I cleaned the filter with tap water, which I have since read is a no no and only rinse every couple of cleaning in old tank water.The filter I have is the Aqueon quiet filer and it says has quiet Aqueon's four-stage filtration process that includes mechanical filtration, chemical filtration, biological filtration, and stationary wet/dry filtration. They are cartridges that can be popped in and out.  They suggest to change the filter every month but I want to keep the good bacteria so I haven't changed it yet.  The conditioner I use is Tetra AquaSafe Plus.
 
Lockman: Ammonia, ammonia.  I hope she is vanquished and the other fish can stay strong. 
 
Sherriff: That ammonia again.  She really is coming back to haunt me huh. I didn't notice she was sick until we were floating the Gourami to acclimate him.  She had been fine for two weeks, and the water parameters as well.
 
Is there a way to test for bacteria infection?
 
Thanks again for all your posts


This old spouse: Yes when we got the last .5 reading, about two weeks ago, we changed daily until we got a zero (testing the next day). Maybe she was too weak by the time we started getting it right.


This is what the filter details state, so I guess the medium is activated carbon.
Stage 1 - Dense floss removes particles and debris.
Stage 2 - Activated carbon removes toxins, odors and discoloration.
Stage 3 - Patented Bio-Holster removes toxic ammonia and nitrites.
Stage 4 - Diffuser grid removes additional toxins while adding oxygen for more active fish. It also reduces splashing makes the return exceptionally quiet.

Yes, she probably was too weak. Sometimes ammonia poisoning doesn't kill them for months. I have an albino cory who survived an ammonia spike who has no barbels and is blind. He keeps hanging on though after more than a year. His partner, a female, is completely intact. 16 others died. Go figure.

How good to read a thread where a new member is supported rather than criticized. I've been keeping fish for over 10 years and all my experience has come either from reading this forum, or making mistakes. Ohhh and there have been many mistakes Keep up the good work jkpedrita and no there isn't a test for bacterial infection, as far as I know.

The lesson to be learned is to try to keep your tank as pristine as humanly possible. The cleaner the tank, the less stressed the fish are and the less likely they are to succumb to disease.