1 sick and 1 dead puffer! HELP!

I had 2 green spotted puffers in a 20 gallon with a specific gravity of 1.008. The are both juveniles hence the small tank and low specific gravity. I have had them both for about a month and a half. Tonight one died on me. The other one looks stressed and ignored some blood worms! Ammonia was slightly high everything else was normal. I did a 75% water change and ammonia is now 0. I was 3 days late on my weekly water change. What do I do? HELP!

He's still not eating. He did refuse snails! The size difference between the two was huge even though they were both juvies. One is about an inch and the one that died was about 1/4th of an inch.

1) I'll test for numbers in a bit. Its raining out and very dark outside so I still have all the fish lights off.
2)20 gallon and 1 green spotted puffer
3)I do a 50% water change weekly. It was last done last night. As I said before I was a few days behind on it. The only tankmates are 2 snails that just refuse to die. One MTS and one ramshorn. I use instant ocean, and prime. I did use cycle when first setting the tank up over a month ago. Nothing else is being used though. They are feed clams, mussles, shrimp (all 3 with shells still on or crushed up some), frozen blood worms, snails (mts, ramshorn, and bladder), and fresh catfish. Everyday they get something different. They still like flakes from being at the LFS so sometimes they got those as well. I feed them the very meaty kind from Kens.
4)No changes at all!
5)Aquarium has been set up for a little over a month. I cycled it when Cycle. Followed the directions to a T and added the puffers a few days later.

Pufferpunk said:

Since Cycle does not cycle a tank & contains nothing but DEAD bacteria, I'm thinking your puffer/s are suffering thru a cycle. I'd be very interested in your water parameters.

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I disagree with you. Cycle does cycle a tank. I have used it on 2 other tanks and tested them every day for 2 months and they remained perfectly stabled and cycled. Just because my posts here are low does not mean that I am new to the hobby. I understand though that there are a lot of new people and assuming that a cycle is going on is always the first best answer. I completely get it. I have someone sleeping on my couch, as soon as they wake up I will turn on the lights and test my water.