sounds like your tank has not cycled yet - which means there is a chemical build up going on in there which is taking out your fish.
You need to understand what the tank cycle is all about, and how to deal with it until things settle down. On a new tank this can take 6 to 8 weeks, so it looks like you're right in the middle of it - bad news for the fish I'm afraid, but it can be handled safely once you know what you're doing.
There are stickies on this site to help you through this, or you can do an internet search on "how to cycle a fish tank"
I'll give you a brief version now so you can see how this pans out - but you should still look it up on other searches even if you understand what I'm about to say.....
Fish poop turns into ammonia - and this builds up until it kills your fish. Fortunately, there is a strain of bacteria that occours naturally that feeds on this ammonia, but that bacteria won't be there in a new tank - it takes a few weeks before it turns up and does its thing. So the first thing you need to do is to get a test kit, check the ammonia level in your tank, and if it gets over 2ppm you need to do a partial water change to dillute it back down to a safe level. With one remaining guppy, the water change could be as little as 20% a week, so it's not a nightmare to do. kinda depends on the size of the tank....
Got your head around that - ok cool - now we move on to the next bit
The bacteria that eats the ammonia turns that ammonia into a chemical called nitrite. Unfortunately this nitrite is just as toxic as the ammonia - so again, you need to do the tests and the water changes to take care of that too, until another form of bacteria come along that eat up the nitrite for you - again this one takes a few weeks to happen, so keep your eyes on the test kit until you have it under control.
Now the easy bit - once you have changed the water enough to keep those levels down to less than 2ppm in both cases, and the bacteria are eating up the ammonia and the nitrite for you (don't forget it takes up to 8 weeks for this to settle down ok) you'll see that your water test kit is telling you that you have no ammonia and no nitrite - woohoo - that's when your tank is "cycled" and your fish will be happy healthy and eating like piggies.
There's no such thing as a free luch however, and so the bacteria that eat the nitite produce another chemical called nitrate. fortunately, this is much less toxic to the fish, and levels up to 30 or even 40ppm are ok - same deal with the control for this one - you do water changes to keep it down to under 30 or 40ppm and everythings fine.
There is no bacteria (for begginers) that eats this nitrate, so it's common practice to just change 20% of the water per week to keep it dilluted
Still with me? - looked up that search?
OK
Your tank is 4 weeks old, and you have one guppy in it. The cycle is half way through now, so you should see ammonia dropping, and nitrite rising. When your tank reads zero for both - your safe.
Do not add any more fish until this is sorted out, or it means more fish poop - more chemicals, and more work with the water changes.
You do not have Ich - forget about that for now. It's the chemical build up that's your problem. ich looks like grains of salt - eg white.
research "fish tank cycle' - change 30% of the tank water - don't add any more fish - and you'll be right soon enough
ask any questions you want - we all went through this when we started, and asking questions and doing reasearch is the best thing you can do - and this forum is the best place to do it in
welcome to the tank
