tricep fatigue when swimming

There's a lot here so I'm going to break it down into separate sections.

You don't specifically say whether the 1500 you refer to was one continuous swim or intervals.  You talk about intervals a bit later on, but it isn't clear how you did the 1500.  In any case, at 2:10/100 you have some significant technique flaws that are holding you back.  Admittedly I have not seen you swim and have no idea what your swimming history is, but I suspect your biggest issues are likely balance in the water (legs dropping), body rotation, and an ineffective catch/EVF.  I base that on 50+ years of swimming experience and 30+ years of coaching experience, that you aren't feeling your lats engaged, and what you say further down about your strokes per length.  

The single best thing you can do is get with a qualified coach/instructor for some lessons.  If that isn't an option, the next best thing is contact a qualified coach/instructor for remote video analysis.  Video isn't as fast as a coach on deck, however it will yield results over a number of weeks (that would involve multiple video, analysis, correction cycles).

It seems regardless of what you do, you end up right around 2:10/100 which tells me that's pretty close to your threshold pace.  While you are sorting your technique out, you might try reducing the interval length and increase your rest interval.  During your workout, you want to strive for PERFECT technique (I know, that's a challenge if you don't have perfect technique).  Your technique is further breaking down as you fatigue.  By shortening the repetition length (maybe 50's instead of 100's) and increasing the rest interval for a few weeks (at least 20" between reps) you will recover and be able to swim longer with proper (or at least better) technique.  You might try something like 30 x 50 @ 1:25 send-off with a target pace of 1:05/50.  That will give you 20" rest after every rep.  Once a week, throw in a set of 4 x 100 @ 2:30 with a target of 2:10.  Can you hold your pace during the 4 x 100 when you do that?  If you can, then you can start alternating 100's one day and 50's the next day.  You DON'T need to be doing long continuous swims.  In the long run, you will be better served doing intervals.  Occasionally you can do a long continuous swim if you feel you need the confidence of having completed the long swim.

Before you say you want to swim faster than 2:10/100, consider this.  You've already said you can't hold anything faster than 2:10/100 for more than one rep.  The reason is you are going anaerobic when you attempt to go faster - your body is essentially not letting you swim faster.  The path to getting faster and swimming at a pace quicker than 2:10/100 is to reduce your swim threshold pace.  You do that by training at or near your threshold (the reason I said target 2:10/100).  In a fairly short period of time your threshold pace will begin to come down.  Once you have good swim fitness and good technique, then you can add in some intensive endurance sets to work on top speed.

I am not big on strokes per length.  That said, it is an indication of your stroke efficiency.  There is no "right" number.  Why?  I'm 6'5" tall.  My "wingspan" - the distance between my hands when my arms are outstretched - is over 7-feet.  I've also been swimming close to 50-years.  If you compare my strokes per length to someone who is 5'5" that's been swimming 1-year, you'll see a SIGNIFICANT difference in our SPL's.  That said, while there isn't a right SPL, I know when it's too low and yours is way too low - meaning your stroke is inefficient.  That's likely a combination of drag and an ineffective catch/EVF leading to an inefficient propulsive phase.

The short answer is that your triceps fatigue isn't the problem, rather it's a symptom of the problem - specifically your technique needs some work.  As I said, the best thing you can do is get with a qualified coach/instructor for some lessons/analysis.

Hope that helps.

Your swimming will thank you!