Tren for cardio athletes is really BAD and doesn't give them any plus just minus.
If you go for Track and Field usually Anavar/Winstrol are the goto along a low dose of Test and EPO.
I know some amateur swimmers here in Italy, that use 250 TestE + EQ/Primo, 100mg Cardio Aspirin (it's an aspirin pill protected by stomach acids, it will open in the intestines and this way you will avoid ulcers), HGH, Humalog/HumalinR post training and EPO (everyone).
2-4iu HGH before bed for recovery.
Insulin usually is used during offseason to replenish faster the muscles after the long training sessions (MMA fighters do that too)
Test + EQ or Primo for anabolism/recovery, a swimmer isn't BIG at all and usually anabolics are low dosed.
EPO is a must, and they spend so much for that, here usually it's stolen from hospitals or some people that does chemio therapy sells it to get some money.
If they only knew that you can buy it Pharma Grade from CHINA LOL!!
To use EPO you need to go by bloods and check CBC levels without EPO and then implement a protocol.
Ofc you can't start with Hemoglobin at 18..... if you don't want to die.
The only one protocol i know is: (it was told me by an athlete himself, but you can find it online too)
1 week- 5k iu EPO first thing am subq divided in 3 shots MWF
2-8 week - you do bloods each week decide if you go for another 5k iu(to raise levels), 2,5k iu(to maintain) or nothing (if too high levels).
Remember that you take EPO to raise Hemoglobin to 17ish g/dl, some aim for more but it's very tricky here.
This swimmer told me that EPO was absolutely the best ped he has ever used.