Fixing a first time Homebrew mistake...

I'd test it and taste it before I diluted it, but Diluting it is a doable thing. Mike 'Tasty' McDole has said that he will take a stronger beer and dilute it down to a session beer. If you do decide to dilute, you're best to boil it first to drive off any O2 and bugs. If I were going to do it, I'd add it in the primary and let it settle for another day or two before bottling to avoid excess O2 exposure. Still If it we're me, I'd be inclined to check the gravity first and if it has fermented down close to your intended FG, I drink it as is. Up to taste really.

 

No, the concentrated wort will not be affected during fermentation as long as you aerated it well before pitching an adequate amount of yeast. However, if you were brewing a high gravity beer, then that step is critical, and fermentation could be incomplete because the yeast may be unable to complete their task if oxygenation was low or the pitch rate was low.

Did you take a gravity reading before and after the boil? How did these readings compare to the expected readings for the recipe?

As slusk says you can still add water up to your 5-gallon measurement, but I'd wait until after fermentation is complete. Then taste the beer, take a gravity reading and compare that reading to the expected final gravity per the recipe. You can expect it to be higher than anticipated since you will have a 'concentrated' beer. It's going to be your choice if you want to add the water. The beer will be flat, but does it taste weak? If so, don't add water, and consider bottling it at that strength because there is no way to make it stronger. Calculate the ABV using your OG and FG readings, and let that also help you decide what you want to do. Brewing a beer to a specific style means that you want each characteristic of the style to be closely met if you are a purist. If your recipe is for a session ale of some sort, then you don't want the ABV to be 8% (for example). Adding a gallon of water would get it down to around 6.

If you do add water, boil it first like slusk mentioned, and siphon it gently into your beer instead of pouring it or you'll introduce more O2 right back into it. Stir it in gently after siphoning to mix it well before you take your 'official' final gravity reading.

One last thing - you don't need to use a secondary fermentor for most beers.

 

The problem could have been solved by taking a gravity reading. If the reading was way too high then you could have add water then. I did the same thing my first brew. I didn't account for the boil off and I started with 5 gallons of water. However, when I checked the gravity reading it was way high.

I would recommended having a better plan laid out so you can make sure you don't miss any steps. Also, drink less beers during brewing and more after brewing. After clean up too, because cleaning up faded or the next day is a total buzz kill.

 

I would skip the secondary, and keep whatever strength you have. You said it's an IPA, so diluting will hurt your hop utilization. And it's your first batch, so just ride with what you have. Feel free to make mistakes and learn from them.

And my biggest mistake in my first batch was drinking during brewing. My rule now is to wait until flameout.

 

My rule now is to wait until morning before starting to drink on brewdays.

(but IANAN)